Military  Topography 

and 

Photography 


BY 

FLOYD  D.  CARLOCK 

United  States  Army 


GEORGE  BANTA  PUBLISHING  COMPANY 

MENASHA,     WISCONSIN 


Copyright  1918 

by 
FLOYD  D.  CARLOCK 


PRINTED  AND   BOUND   BY 

GEORGE   BANTA   PUBLISHING  CO. 

MANUFACTURING  PUBLISHERS 

MENASHA,    WISCONSIN 


TABLE  OF  CONTENTS 

CHAPTER  I 

MILITARY  MAP  READING  PAGE 

Definition  of  Map        ..........         1 

Classes  of  Maps  ..........         1 

Plane  and  Topographical      ........         1 

Civil  and  Military 1 

Military   Maps 1 

Road   Sketches      .         . 1 

Position  Sketches .         .2 

Outpost  Sketches /  .         .         .2 

Place   Sketches 2 

War  Game  and  Fortress  Maps*"  .         .         .         .         .         .         .2 

Elements  of  Topographical  Maps 2 

Direction       .  3 

Methods  of  Expression  ........         3 

Magnetic  Declination      ........         4 

Bearing .         .         .5 

Azimuth  and  Back-Azimuth   .......         5 

Methods  of  Measurement 6 

Distance        ...........         7 

Methods  of  Expression 7 

Methods  of  Measurement       .         .         .         .         .         .         .         8 

Difference   in    Elevation        ........         8 

Methods  of  Expression  ........         8 

Methods  of  Measurement       .         .         .         .         .         .         .9 

Conformation  of  the  Ground 13 

By  Contours .          .13 

By    Hachures 13 

By    Relief '  .13 

Graphic    Representations 16 

Streams,  Lakes,  Etc 18 

Artificial  Constructions 18 

Vegetation .         .       1# 

Scales  of  Maps   . 18 

Methods  of  Expression ,         .       18 

Representative   Fraction .18 

Words   and   Figures 19 

Graphic  Representation  ........       19 

Graphic   Scales      .          .          . 19 

Reading  Scales 19 

Working  Scales 19 

Slope  Scales 19 

Normal  System  of  Scales,  U.  S.  Army 21 

To  Construct  a  Reading  Scale 21 

To  Construct  a  Slope  Scale    .  22 

Orientation  of  Maps   .......  .23 

By  compass.          ........  .24 

By  the  Sun  .  24 


PAGE 

By  the  North  Star 25 

By  Watch 27 

By  Comparisons  ... 27 

Location  of  Map's  Position 28 

By  Two  Plotted   Points 28 

By  Three  Plotted  Points 28 

By  "Ranging  in" 30 

By  "Lining  In" .30 

Visibility  of  Points 31 

Visibility  of  Areas 38 

Map  Interpretation     .         .         .         .         .         .         .         .         .         .38 

Systematic  Reading  of  Maps       .         .         .         .         .         .  ^  .40 

CHAPTER  II 

TOPOGRAPHICAL  SURVEYING 
General  Remarks         ..........       43 

Geodetic  Operations    ..........       44 

Geodesy         ...........       44 

The  Base  Line 44 

Latitude    Determination        .         .         .         .         .         .         .         .46 

Longitude   Determination      ........       48 

Triangulation 48 

Control  Work  of  Military  Surveys 52 

(1)  With  Geodetic  Triangulation  Stations 52 

Preliminary  Reconnaissance   .......       54 

Primary  Triangulation 54 

(2)  Without  Geodetic  Triangulation  Stations       ....       56 

Preliminary  Reconnaissance   .         .         .         .      "   .         .         .57 

Base    Line      ..........       67 

Primary  Triangulation  .         .         .         .         .         .         .         .57 

Secondary    Triangulation      ........  67 

Tertiary    Triangulation          ........  58 

Control   Traverses          .         .         .         .         .         .         .         .         .58 

Triangulation    Leveling .58 

Plane-Table    Operations 58 

Topographic  Methods 59 

Preparation  of  Field  Sheets 60 

Plotting  Coordinate  Lines 62 

Plotting  Triangulation  Stations 62 

Setting  Up  the  Plane  Table 63 

Instrument  Stations 64 

Location  by  Resection 64 

The  Two-Point  Problem 64 

(1)  By  Compass   Orientation 64 

(2)  By  Graphic  Orientation 64 

The  Three-Point   Problem 67 

(1)  By  Compass  Orientation — Coast  Survey  Solutions  67 

(2)  By  Graphic  Orientation— Bessel's  Method  .         .  73 

(3)  By    Mechanical    Orientation        ....  73 

"Ranging    In".         .         ., 75 

"Lining    In" 75 

The  One-Point  Problem  .  .  76 


PAGE 

Location  by  Meandation          .......       78 

Critical  Points  .  .  .  ..  .  .  .  .  .79 

Location  by  Radiation— Stadia 79 

Location  by  Intersection 80 

Location  by  Resection 81 

Location  by  Meandation  .  .  .  .  .  .  .81 

Elevation  Determination  of  Instrument  Stations  and  Critical  Points  81 

In  Resection 82 

In  Meandation 83 

In  Radiation 83 

In  Intersection  .  .83 

Sketching  Operations .84 

Plotting  Direction 84 

With  Alidade  Ruler 84 

With  T-Square  and  Protractor 84 

With  Paper  Protractor  and  Triangles 85 

Plotting  Distance  .  .  --.  .  .  .  .  .86 

With  Reading  Scales 86 

With  Strip  of  Blank  Paper 86 

With  Dividers 86 

Plotting  Slopes  ....  .  .88 

Even  Slopes .89 

Convex  Slopes 90 

Concave    Slopes      .         .         .         .         .         .         .         .         .91 

Changes  Between  Slopes 91 

Plotting  Character  of  the  Terrain 91 

Natural  Character  of  the  Terrain 91 

Vegetation ...  93 

Lines  of  Communications 93 

Towns  and  Buildings 94 

Sketching,  Concrete  Example  of  . 94 

Designation  of  Stations  . 94 

Sketching — Procedure 96 

Title  and  Authorship 98 

Field  Notes .100 

Photo-Topographic  Operations  .  .  .  .  .  .  .  .  102 

General    Principles        .........     102 

Photo-Topographic   Instruments    .......     102 

Camera  and  Accessories  .......  102 

Photo-Theodolite 102 

Stereo-Comparator.          ........     104 

Stereo-Autograph  .........  104 

Photo-Topographic  Control 104 

Photographic  Intersection 109 

Without  Orientation .109 

With  Orientation .112 

Photographic  Resection 113 

The  Three-Point  Problem 113 

The  Five-Point  Problem 115 

Orientation  of  Picture  Trace— Schniffer's  Method  .  .  107 

Distance  Determination  by  Stereo-Comparator  .  .  .118 
Elevation  Determination  .  121 


CHAPTER    III 

MILITARY    SKETCHING  PAGE 

Measuring  and   Plotting  Direction      .  .  .  124 

With  Prismatic   Compass      ...  .     124 

With  Sketching   Board  Oriented  .  .      125 

Measuring  and  Plotting  Distance       ...  ...     126 

By    Pacing,    Timing,    Etc 126 

By    Intersection    .         .         .         .         .     .    .         .         .         .         .     127 

By  Resection .         .          .127 

By  Estimation      .....  .     127 

•Construction  of  Working  Scales  ...  .  .     128 

Measuring  and  Plotting  Slopes  .          . 131 

Vertical   Angle    Measurements      .          .          .          .          .          ...      131 

Elevation  with  Aneroid  Barometer       ......      132 

Stadia    Reductions 132 

Use   of  Slope  Scales .133 

Plotting  Slopes .133 

Plotting  Character  of  Terrain .133 

Streams,  Lakes,  Etc.   .....  .133 

Vegetation .135 

Lines  of  Communication .     135 

Towns  and  Buildings .135 

Classes   of   Sketches .         .         .135 

Road  Sketches 135 

Position    Sketches          ......  .135 

Outpost  Sketches .135 

Place  Sketches .135 

Kinds   of   Sketching .135 

Individual  Sketching 135 

Individual   Road   Sketching .135 

Back-sight   Method  .  .     135 

Compass   Method      .          .  .137 

Individual    Position    Sketching 141 

Individual  Outpost  Sketching .      143 

Place  or  "Eye"  Sketching .143 

Memory    Sketching          .          .          .          .          .          .          .          .      144 

Combined  Sketching      ..........      144 

Combined  Road  Sketching 145 

Combined  Position  Sketching 147 

Combined  Outpost  Sketching 150 

Rapid  Sketching— General  Principles  152 

Knowledge  of  Map   Distances      .......     153 

Knowledge  of  Ground  Distances  .......     154 

Knowledge    of    Directions    ........     154 

Knowledge  of  Slopes     .          .         .          .          .          .         .         .154 

Accuracy   and    Neatness 154 

CHAPTER  IV 
PHOTOGRAPHY 

The    Camera 156 

The  Box .156 

Of   Fixed   Focus   Cameras    .          .  ' 156 

Of  Focusing  Cameras  .          .          .          .          .          .          .          .          .     156 


PAGE 

The   Lens 158 

Optics  of  Lenses 159 

Focal    Length 159 

Speed  of  Lenses     .........  160 

Depth  of  Focus 161 

Definition 161 

Astigmatism  .          .          .          .          .          .                    .          .          .  162 

Styles  of  Lenses 162 

Single    Lenses 162 

Meniscus  Form 162 

Piano-Convex          .         .         .         .          .         .         .         .         .162 

Achromatic  Single  Lenses     .         .         .         .         .         .         .         .  162 

Rapid  Rectilinear  Lenses     ........  163 

Anastigmat   Lenses 164 

The  Shutter— General  Principles 165 

Styles    of   Shutters        .         . .166 

Simple    Shutters     .         .     - 166 

Automatic    Shutters       ........  166 

Focal  Plane  Shutters     .         .         .         .         .         .         .         .167 

Control  and  Care  of  Shutters 167 

The  Dry  Plate 168 

Plate    Holders 169 

Roll  Films .170 

Film   Packs 170 

Styles  of  Cameras      . 170 

Using    the    Camera    .         .         .          .         .         .         .         .         .         .  172 

Position  for  Exposure           .          .         .          .  t       .         .         .         .  172 

Support   for    Exposure          ........  173 

Adjustment   for   Exposure    ........  173 

Focusing — Judging    Distances      '.......  174 

Timing  the  Exposure    .         .         . 174 

Developing  ..........         .-  174 

The  Dark  Room 174 

Lighting  the  Dark  Room      .     - ,         .         .174 

Water  Supply       .                   .                   175 

Arrangement  of  Apparatus 175 

Cleanliness    . 176 

The   Developer      .          .'-...         .- 176 

Developing  in  Dark  Room  ........  178 

Normal    Procedure .         .178 

Overexposed    Plates .  179 

Underexposed   Plates    ....  .179 

Daylight  Developing— Film  Tank  Developer 179 

The  Fixing  Bath  and  Fixing     . 186 

Washing  and  Drying  .         .                   .         .         .         .         .         .         .  187 

Printing 187 

Printing  Papers    .         .         . 187 

Printing   Light  and   Exposure      .......  188 

Developing    and    Fixing       ........  189 

Washing  and  Drying    .........  190 

Mounting      ...........  190 

Intensification  and  Reduction      ........  191 

Certain    Photographic   Terms 191 


PAGE 

Defects  in  Negatives 192 

Too  "Thin" 192 

Too    "Dense" 192 

Much  Contrast  but  Little  Detail  in  Shadows         .         .         .         .  192 

Little  Contrast  but  Much  Detail  in  Shadows     ....  192 

"Fogged"   Negatives 192 

Lack    of    Sharpness 192 

Spreading  of  High  Lights 193 

Black  Streaks  or  Blotches   .         .         .         . ,  .         .         .193 

Frosty  Appearance 193 

Finger  Marks       . 193 

Stains 193 

Pin  Holes 194 

Transparent  Spots 194 

Opaque  Spots 194 

Transparent   Lines        .         .         .         .         .         .         .         .         .194 

Opaque  Lines       ..........  194 

Mottled    Appearance 195 

Defects   in   Prints 195 

Military    Photography 196 

Position   and   Outpost  Views 196 

Place   and    Reconnaissance   Views 196 

Topo-Photography .         .         .         .  196 

Aero-Photography 196 

General    Principles 196 

Rectification  of  Distortion 197 

The  Scheimpflug  Camera 198 

The    Scheimpflug-Kammerer    Perspectograph        ....  199 

CHAPTER  V 
SPECIAL  PROBLEMS 

Chaining 206 

Telemetric  Measurement  of  Distance 210 

Theory  of  the  Stadia  .         . 210 

Inclined    Readings 212 

Verniers 213 

Direct  Verniers 213 

Retrograde  Verniers 214 

Least   Count   of   Verniers .         .215 

Double    Verniers 215 

Folded    Verniers 216 

Adjustments  of  the  Level 216 

Adjustments  of  the  Transit         .         . 217 

Adjustments  of  the  Plane  Table         .         .                  .         .         .         .  217 

Spirit  Leveling .221 

General  Discussion       .         .         .         .                  .         .         .         .  223 

Leveling   Terms 223 

Leveling — Procedure     .         . 225 

Field  Notes 220 

Limit   of   Error 226 

Standardization  of  Tape  or  Chain        .     ' 226 

To  Construct  a  Base  of  Standardization                                              .  227 


PAGE 

Determination  of  the  Constants  of  a  Tape 228 

Of  Temperature 228 

Of  Tension 228 

Of  Sag                   228 

Determination  of  Stadia   Constant 229 

Control  Traverse .         .  230 

Procedure 230 

Field  Notes .234 

Traverse  Sheet 236 

Back-Sight  Traverses           .........  237 

Needle  Traverses 237 

Measurement   of    Base   Line,   1:100,000 238 

Record  of  Base  Line  Measurements     ......  239 

Base   Line   Computations      ........  239 

Measurement  of  Angles  in  Series        .......  241 

Record  of  Angles  Measured  in  Series 241 

Measurement  of  Angle  by  Repetition 241 

Record  of  Angles  of  Triangles  Measured  by  Repetition          .         .  244 

Azimuth  by  Polaris  Observation  . 246 

Azimuth  by  Sun  Observation 248 

Astronomical    Terms    .........  248 

Time 249 

General    Principles      ...                                     ...  250 

Parallax   and    Refraction 251 

Declination  .         .         .         .         . 251 

Method  of  Observation           .         . 252 

Field   Notes  of   Sun  Azimuth 254 

Sun  Azimuth  Computation 255 

Latitude,  Longitude  and  Azimuth  Determinations  by  the  Solar  Attach- 
ment         256 

General  Explanation  of  the  Solar   Attachment    .          .         .         .  256 
To  Find  the  Latitude  at  Apparent  Noon     .                  .         .         .258 

To  Find  the  Longitude  at  Apparent  Noon 260 

To  Find  the  True  Azimuth  at  Apparent  Noon    .         .         .         .261 

Determinations  at  Other  Times  than  Apparent  Noon     .      .    .         .  262 

To  Compute  the  Declination  of  the  Sun 262 

Accuracy  and  Use  of  the  Solar  Attachment  .          .         .         .         .  263 

Adjustments  of  Solar  Attachment 263 

Adjustments  of  the  Telescopic  Solar 265 

Map  Reproduction  and  Enlargement    .......  265 

Map  Reproduction      .         .         .         .         ...         .         .         .  265 

Tracing 265 

Carbon  Copying     .         .  '       .         .         .         ..        .          .         .  265 

Free  Hand 265 

Blue  Printing          .         .         . 266 

White  Printing 266 

Photographic  Method 267 

Lithographic   Methods 267 

Map  Enlargement    ...                   267 

By  Pantagraph 267 

Photographic  Method 267 

By  Coordinate  Lines 268 

Poly  conic  Projections  .         .         . 268 


CHAPTER  VI 

CONVENTIONAL    SIGNS  PAGE 

Works  and  Structures         .          .          .          .          .          .          .          .          .  272 

Boundaries,  Marks  and  Monuments     .......  276 

Drainage 277 

Relief— Contours          ...                                                                   .  278 

Land  Classification .                                      .  279 

Hydrography,  Dangers  and  Obstructions     .          ...          .          •          •  283 

Aids  to  Navigation      .......                             .  287 

Special  Military  Symbols 289 

Lettering .      .  291 

Authorized  Abbreviations 293 

TABLES 

TABLE    I.    Conversions 296 

TABLE    II.    Trigonometric    Formulae 297 

TABLE  III.  Logarithms  of  Numbers 299 

TABLE   IV.  Stadia  Reductions  for  100         ..                    ...  301 
TABLE   V.    Polyconic   Projections          .          .          .          .                    .          .303 


PREFACE  TO  SECOND  EDITION 

In  writing  this  book,  it  has  been  the  aim  of  the  author  to 
present  the  different  subjects  and  their  expositions  in  a  logical, 
clear,  and  precise  way,  and,  at  the  same  time,  to  cover  the  whole 
completely  and  comprehensively. 

Topographical  sketching  can  not  be  learned  from  books 
alone.  In  fact,  the  function  of  the  book  is  to  present  the  prin- 
ciples in  a  logical  and  intelligent  manner  which  are  to  govern 
topographical  operations  in  the  field.  Facility  and  precision 
in  applying  those  principles  can  be  acquired  only  through 
practice  and  experience.  By  the  study  of  and  reference  to  this 
manual  the  beginner  will  be  enabled  to  plan  and  start  and  carry 
through  intelligently  any  task  in  topography  that  may  be 
assigned  to  him,  whether  it  be  an  individual  or  combined  sketch, 
or  an  extended  topographical  survey.  Only  by  those  to  whom 
the  latter  task  has  been  assigned  can  a  clear  knowledge  of  these 
requirements  be  appreciated. 

Photo-Photography  and  Elementary  Photography  have 
been  covered  sufficiently  to  serve  as  a  complete  guide  for  those 
who  are  or  may  be  provided  with  photo-topographic  surveying 
instruments.  The  study  of  these  subjects  may,  of  course,  be 
omitted  by  one  not  interested  in  them,  as  the  subjects  of  Military 
Topography  and  Sketching,  and  Map  Reading  are  complete  in 
themselves  and  in  no  way  based  on  the  former. 

Geodetic  methods,  adjustments,  and  computations  of  lati- 
tude, longitude,  and  spherical  triangles  have  not  been  included  in 
the  chapter  on  "Special  Problems,"  for  the  military  topographer 
or  sketcher  will  never  be  confronted  with  those  operations. 
These  subjects,  however,  have  been  treated  sufficiently  in  the 
text  to  enable  him  to  take  advantage  of  geodetic  data  of  areas 
he  may  be  required  to  survey  or  sketch. 

The  method  of  Combined  Sketching  covered  in  Chapter  III, 
was  conceived  and  developed  by  Colonel  E.  R.  Stuart,  U.  S. 
Military  Academy,  to  whom  the  Service  is  greatly  indebted  for 
the  same. 


Preface 

The  author  wishes  to  acknowledge  his  indebtedness  and  to 
express  his  appreciation  and  thanks  to  the  Superintendent  of 
the  Coast  &  Geodetic  Survey,  to  the  Director  of  the  Geological 
Survey,  to  the  Superintendent  of  the  U.  S.  Naval  Observatory, 
to  the  Eastman  Kodak  Co.,  to  W.  &  L.  E.  Gurley,  to  Carl 
Zeiss  of  Jena,  and  to  Th.  Scheimpflug  of  Vienna,  for  their 
manuals,  texts,  reports  and  other  publications  with  which  they 
have  so  considerately  furnished  him.  Also,  for  the  many  electro 
plates  and  half  tones,  acknowledgments  of  which  have  also  been 
made  with  the  insertions  of  the  same. 

The  author  wishes  also  to  express  his  indebtedness  to  General 
H.  C.  Hodges,  Jr.,  to  Major  J.  W.  Heard,  and  to  the  late 
Captain  Roderick  Dew,  who  assisted  him  in  the  proof  reading 
of  the  first  edition,  for  the  invaluable  help  they  rendered  him. 
;[•  If  ,  !i  F.  D.  C. 

Chickamauga  Park,  Georgia, 
December,  1917. 


CHAPTER  I 

MILITARY  MAP  READING 

\ 

DEFINITION  OF  MAP.  A  map  is  a  representation-  of  the 
earth's  surface,  or  a  portion  of  it,  showing  the  relative  size  and 
position  of  the  parts  represented  according  to  some  given  scale 
or  projection.  The  representation  is  usually  on  a  plane 
surface. 

CLASSES  OF  MAPS.  All  maps  may  be  divided  into  two  general 
classes — plane  and  topographical.  Plane  maps  include  cadas- 
tral and  all  other  maps,  showing  the  meets  and  bounds  of 
private  and  public  property,  the  boundaries  of  governments, 
location  of  roads,  rivers,  etc. :  in  brief,  all  maps  in  which  the 
horizontal  coordinates  only  of  the  parts  represented  are  shown. 
Topographical  maps  include  all  those  in  which  the  conformation 
of  the  ground  is  represented.  This  conformation  may  be  repre- 
sented by  relief,  by  hachures,  or  by  contour  lines. 

Maps  may  also  be  divided  into  civil  and  military  maps, 
according  to  the  uses  for  which  they  are  intended  or  put.  All 
maps  are  of  some  military  use,  but  military  maps  proper 
include  only  those  which  have  been  made  for  military  operations 
and  study.  Such  maps  are  much  more  complete  than  is  re- 
quired of  other  information  and  cadastral  maps.  In  addition 
to  a  more  accurate  representation  of  the  conformation  of  the 
ground,  they  show  the  extent  and  character  of  the  vegetation, 
etc.  Military  maps  of  the  United  States  Army  are  divided  into 
three  general  classes:  (1)  Road  Sketches,  (2)  Area  Sketches, 
and  (3)  War  Game  and  Fortress  Maps. 

ROAD  SKETCHES.  Road  sketches  are  rapid  topographical 
sketches  of  roads  and  trails  and  the  adjacent  terrain.  They 
may  be  either  (a)  Individual  Road  Sketches,  or  (b)  Combined 
Road  Sketches.  The  former  are  of  limited  extent,  generally 
of  a  single  road,  and  executed  by  one  person:  the  latter  cover 
a  network  of  roads  and  are  executed  by  several  persons  sketch- 
ing simultaneously. 


2  Military  Topography  and  Photography 

AREA  SKETCHES.  Area  sketches  are  rapid  topographical 
surveys  of  sections  of  the  terrain.  Rapid  military  topographi- 
cal surveying  is  called  "military  sketching"  and  should  be 
distinguished  from  precise  military  topographical  surveying, 
usually  referred  to  as,  "military  topographical  surveying,"  or 
simply  "military  surveying."  Area  sketches  are  divided  into 
three  general  classes:  (a)  Position,  (b)  Outpost,  and  (c)  Place 
Sketches. 

POSITION  SKETCHES.  Position  sketches  are  rapid  military 
topographical  surveys  of  sections  to  which  the  sketchers  have 
access.  They  may  be  either  (a)  Individual  Position  Sketches, 
or  (b)  Combined  Position  Sketches,  according  as  to  whether 
they  are  made  by  single  sketchers,  or  by  several  sketchers  work- 
ing simultaneously  and  in  conjunction. 

OUTPOST  SKETCHES.  Outpost  sketches  are  rapid  military 
surveys  of  sections  along  the  outposts  and  as  far  towards  the 
hostile  position  as  can  be  sketched  from  the  line  of  observation. 
Similar  to  position  sketches,  they  may  be  divided  into  (a) 
Individual,  and  (b)  Combined  Outpost  Sketches. 

PLACE  SKETCHES.  Place  sketches  are  rapid  military  topo- 
graphical surveys  of  sections  made  from  but  one  point  of 
observation.  They  may  be  either  (a)  Eye  Sketches,  or  (b) 
Memory  Sketches.  Eye  sketches  are  those  place  sketches  of 
areas  in  which  *he  sketcher  was  able  to  complete  his  map  while 
remaining  at  a  single  point  of  observation.  Memory  sketches 
are  those  in  which  the  terrain  is  sketched  from  memory.  Often 
the  only  maps  of  the  terrain — of  its  roads  and  trails,  within 
hostile  areas  will  be  "Memory  Sketches." 

WAR  GAME  AND  FORTRESS  MAPS.  These  are  precise  military 
topographical  surveys  of  limited  areas,  or  of  fortresses,  and 
used  principally  for  map  maneuvers  and  study.  They  include 
field  maneuver  maps. 

ELEMENTS  OF  TOPOGRAPHICAL  MAPS 

RELATIONS  OF  POINTS  ON  THE  TERRAIN.  For  every  two 
points  on  the  terrain  there  are  always  three  and  only  three 
relations  between  them:  (1st)  the  relation  of  direction;  (2nd) 
the  relation  of  distance;  and  (3rd)  the  relation  of  elevation. 


Military  Topography  and  Photography  3 

That  is,  one  point,  with  respect  to  another,  lies  in  a  certain 
direction  from  it,  is  a  certain  distance  from  it,  and  is  of  the 
same  elevation  or  is  so  much  higher  or  lower  in  elevation. 

i    DIRECTIONS 

METHOD  OF  EXPRESSION.  In  order  to  express  the  relation 
of  direction  between  two  points,  it  is  necessary  to  assume  one 
point  as  a  base  point  and  to  designate  the  other  point  with 
respect  to  it.  It  matters  not  which  point  is  selected  as  the 
base  point.  Thus,  of  two  points,  A  and  B,  on  the  terrain,  we 
may  select  point  A  as  the  base  and  say  the  direction  of  point  B 
is  north ;  or  we  may  select  poiitt  B  as  the  base,  and  say  that  the 
direction  of  point  A  is  south.  Direction  is  usually  designated 
by  the  points  of  the  compass ;  as,  north,  west,  northeast,  south, 


4  Military  Topography  and  Photography 

southeast,  etc.  In  survey  work  a  more  precise  method  of  direc- 
tion designation  is  necessary  and  circular  measure  is  used.  In 
using  circular  measure  for  the  designation  of  direction,  it  is 
necessary  to  .have  a  point  of  reference  for  the  zero  of  the 
circle  or  scale.  This  point  may  be  the  true  north  or  south,  or 
the  magnetic  north  or  south.  The  point  on  the  terrain  taken 
as  the  base  point  is  considered  as  the  center  of  an  imaginary 
horizontal  circle,  the  zero  of  which  circle  is  considered  the 
true  north  or  south,  or  the  magnetic  north  or  south,  and  the 
imaginary  line  joining  the  base  point  and  the  zero  of  the 
imaginary  circle,  the  line  of  reference;  and  the  line  joining  the 
base  point  with  the  point  whose  direction  is  to  be  designated, 
the  line  of  direction.  The  line  of  reference  and  the  line  of 
direction  form  an  angle  with  each  other  at  the  base  point  whose 
magnitude  can  be  accurately  expressed  in  circular  measure. 
The  direction  between  two  points  is  expressed  in  surveying  as 
the  angle,  in  circular  measure,  which  the  line  of  direction  makes 
with  the  line  of  reference ;  also,  as  the  azimuth  or  bearing. 

MAGNETIC  DECLINATION.  The  magnetic  needle  points  to- 
wards the  north  only  in  a  few  places  on  the  surface  of  the 
earth,  in  most  places  it  points  more  or  less  away  from  the 
true  north,  even  as  much  as  20°.  The  angle  which  the  mag- 
netic needle  makes  with  a  true  north  and  south  line  at  any  point 
of  the  earth's  surface  is  called  the  magnetic  decimation  for  that 
point.  This  magnetic  declination  varies  not  only  with  the 
longitude,  but  also  with  the  latitude.  The  variation,  however, 
is  not  constant  for  the  same  longitude  or  latitude,  so  that  the 
lines  connecting  points  of  equal  magnetic  declination  are  very 
irregular  in  curvature.  Such  lines  are  called  isogonic  lines. 
Lines  of  no  magnetic  declination  are  called  agonic  lines. 

The  magnetic  declination  at  any  point  is  not  constant. 
There  are  cyclic  variations  throughout  the  day,  the  month,  and 
the  year.  There  is  also  a  progressive  variation,  which  appears 
to  be  periodic  in  its  character,  like  the  motion  of  a  pendulum, 
but  which  takes  several  hundred  years  for  its  cycle.  This 
secular  variation  at  any  point  on  the  earth  slowly  increases 
from  zero  to  maximum  in  one  direction;  it  then  slowly  de- 
creases to  zero  and  increases  to  a  maximum  in  the  other 


Military  Topography  and  Photography  5 

direction,  and  so  on.  In  addition  to  the  above  variations,  there 
are  variable  variations  due  to  magnetic  disturbances,  to  storms, 
and  to  magnetic  attractions.  Electric  and  steam  railways, 
telephone  and  telegraph  lines,  pipe  lines,  etc.,  are  sources  of 
large  variable  variations,  while  any  steel  object  near  a  compass 
will  affect  the  magnetic  needle  to  some  extent.  Iron  ores  which 
contain  magnetite  or  pyrrhotite  affect  the  magnetic  needle 
also.  Lunar  and  annual  variations  may  be  neglected,  and  also 
the  diurnal  variation  for  military  work.  The  secular  and 
irregular  variations,  however,  must  be  allowed  for.  Charts 
showing  isogonic  lines  for  the  year  may  be  obtained  from  the 
Government  Printing  Office*,  Washington,  D.  C.  Where  the 
magnetic  conditions  are  so  uncertain  as  to  make  the  direction 
determined  by  it  unreliable,  other  methods  must  be  used  to 
determine  directions. 

BEARING.  If  the  line  of  reference  be  extended  beyond  the 
base  point,  then  the  line  of  direction  will  form  two  angles  with 
it,  which  are  supplements  to  each  other,  and  the  smaller  of  the 
two  angles  is  called  the  bearing  of  the  direction  point  with 
respect  to  the  base  point.  The  bearing  of  a  point  or  line  is 
never  over  90°,  and  it  is  always  measured  from  the  north  or 
south  towards  the  east  or  west.  Whenever  the  line  of  reference 
is  a  true  north  and  south  line,  the  bearing  of  any  point  or  line 
with  respect  to  it,  is  called  the  true  bearing;  whenever  the  line 
of  reference  is  the  magnetic  decimation  of  the  base  point,  the 
bearing  of  any  point  or  line  with  respect  to  it,  is  called  the 
magnetic  bearing. 

AZIMUTH.  The  azimuth  of  a  line,  or  of  a  point  with  respect 
to  another  point,  is  the  angle  which  it  makes  with  the  line  of 
reference,  measured  clockwise  from  the  true  or  magnetic  south. 
Whenever  the  true  south  is  used  as  the  zero  of  the  circle,  the 
angle  is  called  the  true  azimuth;  whenever  the  magnetic  south 
is  used,  the  angle  is  called  the  magnetic  azimuth.  When  the 
word  azimuth  is  used,  the  true  azimuth  is  usually  intended,  and 
when  the  word  bearing,  the  magnetic  bearing  is  usually  in- 
tended. 

The  back-azimuth  of  a  line  (or  of  a  point  with  respect  to 
another)  is  the  angle  which  the  line  makes  with  the  true  or 


6  Military  Topography  and  Photography 

north  and  south  lines  passing  through  the  other  extremity 
of  the  line.  The  difference  between  the  true  azimuth  and  the 
true  back-azimuth  of  a  line  is  never  exactly  180°,  except  when 
the  true  azimuth  is  0°  or  180° ;  for  all  other  azimuths  the 
difference  is  180°  plus  or  minus  a  small  increment  or  decre- 
ment. This  is  due  to  the  convergence  of  the  terrestrial  meridians 
which  are  the  only  true  north  and  south  lines.  For  a  line 
running  directly  east  and  west,  this  difference  will  amount  to 
about  one  minute  for  every  five  miles. 

The  magnetic  needle  points  at  the  true  north  pole  and  at  the 
north  magnetic  pole  in  only  a  few  places  on  the  earth.  This  is 
due  to  the  fact  that  the  magnetic  north  is  not  at  the  true 
north  pole,  and  second,  that  the  lines  of  magnetic  forces  which 
run  from  one  magnetic  pole  to  the  other  are  not  straight  lines. 
Isogonic  lines,  although  of  irregular  curvature,  are  not  mag- 
netic lines  of  force,  but  merely  lines  joining  those  points  of  the 
earth  having  the  same  magnetic  declination  and  running  in  a 
general  north  and  south  direction.  The  reader  must  thoroughly 
clear  this  matter  up  in  his  own  mind. 

METHODS  OF  MEASUREMENT.  In  circular  measure,  angles 
are  measured  in  degrees,  minutes,  and  seconds.  The  azimuth 
of  a  line  is  always  measured  from  the  south  and  in  a  clockwise 
direction.  The  azimuth  of  the  west  is  therefore  90°,  of  the 
north  180°,  of  the  east  270°,  and  of  the  south  0°,  and  so  on. 
Bearings  are  measured  from  both  the  north  and  the  south  and 
in  a  clockwise -or  counter-clockwise  direction  according  to  the 
quadrant  in  which  the  bearing  lies.  The  true  north  is  usually 
designated  as  "due  north";  south,  "due  south";  east,  "due 
east";  west,  "due  west";  northeast,  "north,  45°  east,"  etc.  It 
is  thus  seen  that  in  azimuths,  the  circle  or  scale  is  divided  into 
360° ;  while  in  bearings,  the  circle  is  divided  into  four  equal 
quadrants  of  90°  each,  the  north  and  south  being  designated  as 
0°  respectively,  and  the  east  and  west  as  90°  respectively. 

In  addition  to  circular  measure,  the  direction  of  a  point  may 
be  designated  or  measured  by  giving  its  horizontal  coordinates. 
For  example,  we  might  say  that  point  A  is  10  miles  north  and 
five  miles  east  of  point  B.  .This  method  is  very  little  used. 


Military  Topography  and  Photography 


DISTANCE 

METHOD  OF  EXPRESSION.     Distance  is  the  space  or  interval 
between  two  points,  and  it  is  expressed  in  terms  of  some  unit 


*MAP  MEASURE 

of  length,  such  as,  feet,  yards,  leagues,  meters,  kilometers,  etc. 
The  distance  between  two  plotted  points  on  a  map  is  called 
their  map  distance,  and  it  is  expressed  in  inches,  centimeters, 

*Courtesy  of  W.   &  L.  E.   Gurley. 


8  Military  Topography  and  Photography 

etc.,  or  in  terras  of  their  actual  ground  distance  apart,  such  as, 
feet,  yards,  miles,  etc. 

A  straight  line  is  the  shortest  distance  between  two  points, 
and  in  plane  surveying  is,  therefore,  the  line  of  distance.  In 
geodetic  surveying,  however,  where  the  curvature  of  the  earth 
must  be  taken  into  consideration,  the  distance  between  two 
points  is  the  arc  which  they  intercept  on  the  great  circle  passing 
through  them.  The  great  circles  of  the  earth,  as  it  is  well 
known,  are  not  perfect  circles ;  for  the  form  of  the  earth  is  that 
of  an  ellipsoid  of  revolution.  The  magnitude  in  lineal  measure- 
ments of  the  units  of  latitude  and  longitude  are  computed  in 
the  United  States  from  the  Clark's  Ellipsoid  of  1866.  The 
curvature  of  the  earth  is  7.92  inches  per  mile  and  varies  as 
the  square  of  the  distance.  This  curvature  must  be  allowed  for 
in  taking  level  sights  of  any  great  distance. 

METHODS  OF  MEASUREMENT.  There  are  three  general 
methods  used  to  measure  distance:  (1st)  by  chaining,  in  which 
a  surveyor's  chain  or  steel  tape  is  applied  to  the  distance; 
(2nd)  by  telemeter,  in  which  the  distance  is  determined  from 
the  magnitude  of  the  intercept  of  two  stadia  wires  on  a  stadia 
rod,  which  intercept  is  directly  proportional  to  the  distance ; 
and  (3rd)  by  triangulation,  in  which  the  distance  is  computed 
from  the  triangle  which  that  distance  forms  with  two  other 
lines,  the  length  of  one  of  the  latter  sides  and  the  sizes  of  the 
angles  of  the  triangle  being  known.  This  from  the  trigonometric 
fact  that  if  one  side  and  the  angles  of  a  triangle  are  known, 
the  other  two  sides  may  be  computed  from  them. 

DIFFERENCE  IN  ELEVATION 

METHODS  OF  EXPRESSION.  The  elevation  of  a  point  is  its 
vertical  distance  above  a  datum  plane,  or  when  spoken  of  with 
respect  to  the  earth  in  geographical  and  geodetic  purposes,  the 
distance  above  the  terrestrial  ellipsoid  of  revolution  determined 
by  sea  level.  Difference  in  elevation  between  two  points,  is, 
therefore,  the  difference  between  their  vertical  distances  above 
the  same  datum  plane,  and  may  be  expressed  either  by  the 
difference  in  feet  or  meters,  or  by  the  slope.  In  a  general 
sense,  the  slope  may  be  defined  as  the  angle  which  the  line  join- 


Military  Topography  and  Photography  9 

ing  the  two  points  of  different  elevations,  makes  with  a  horizon- 
tal line  in  the  same  vertical  plane.  This  word,  however,  is  used 
both  in  a  descriptive  and  quantitative  sense.  In  the  former, 
the  slope  of  the  ground  between  two  points  is  described  as  uni- 
form, convex,  concave,  etc. :  in  the  latter,  the  degree  of  the 
slope  is  expressed  either  in  percentage,  in  gradient,  or  in  degrees. 

The  percentage  of  a  slope  is  the  ratio  between  the  difference 
in  elevation  between  the  extremities  of  a  slope  and  their  horizon- 
tal distance ;  or  decimally  expressed,  the  per  cent.  The  gradient 
is  the  ratio  between  the  difference  in  elevation  between  the 
extremities  of  a  slope  and  th^ir  horizontal  distance,  fractionally 
expressed.  The  degree  of  a  slope  is  the  angle  between  the  slope 
of  the  ground  and  a  horizontal  line  in  the  same  vertical  plane. 

METHODS  OF  MEASUREMENT.  The  height  of  a  column  of 
mercury  varies  inversely  with  the  elevation,  so  that  a  scale 
properly  graduated  to  indicate  the  height  of  the  Column  at 
different  elevations,  may  be  used  to  determine  elevations.  The 
mercury  column  is  so  non-portable,  that  it  has  no  practical 
value  as  an  instrument  to  determine  elevations.  The  Aneroid 
Barometer  which  in  size  and  shape  resembles  a  watch,  is  very 
portable  and  is  much  used  in  determining  elevations.  In  this 
barometer,  the  column  of  mercury  is  replaced  by  a  thin  metal 
diaphragm  which  receives  the  atmospheric  pressure  on  one 
side  and  indicates  it  on  a  scale.  In  view  of  the  fact  that 
temperature,  humidity,  etc.,  vary  the  pressure  of  the  atmos- 
phere, the  barometer  can  be  used  only  to  determine  the  differ- 
ence in  elevation  between  two  points  occupied  closely  in  succes- 
sion, and  not  absolute  elevations.  This  for  field  sketches  may 
be  taken  from  the  following  equation : 

E=[e  —  e']  +  [(t  +  t')--H)0] 

Where  E  equals  the  true  difference  in  elevation;  e  and  t,  the 
barometric  reading  in  feet  and  temperature  (Fahr.)  at  higher 
station ;  and  e'  and  t',  the  barometric  reading  in  feet  and 
temperature  (Fahr.)  at  lower  station — assuming  the  two  sta- 
tions to  have  been  occupied  in  succession  as  rapidly  as  possible. 
The  spirit  level  is  the  most  precise  way  in  measuring  eleva- 
tions. A  line  of  levels  is  run  between  the  two  points  which 


10  Military  Topography  and  Photography 

should  give  the  true  difference  in  elevation  between  them. 

If  the  distance  between  two  points  be  determined  by  the 
stadia  and  the  vertical  angle  be  read  at  the  same  time,  the 
difference  in  elevation  can  be  computed  from  the  trigonometric 
formula :  that  the  difference  in  elevation  is  equal  to  the  tangent 
of  the  slope  angle  times  the  horizontal  distance. 

By  Contour  Intervals:  A  contour  is  the  line  of  intersection 
of  a  horizontal  plane  with  the  slope  of  the  ground.  More 
specifically,  it  is  the  line  of  intersection  of  concentric  ellipsoids 
of  revolution  with  the  slope  or  surface  of  the  earth.  The 
ellipsoid  of  revolution  determined  by  the  surface  of  the  sea 
is  taken  as  the  zero  contour.  In  order  to  show  the  conforma- 
tion of  the  ground  contour  lines  are  drawn  for  every  five,  ten, 
twenty,  sixty,  .  .  .  feet  of  elevation,  according  to  the 
scale  of  the  map.  Contour  lines  may  be  easily  understood  by 
imagining  the  sea  to  rise  twenty  feet,  then  the  new  shore  line 
would  represent  the  twenty  foot  contour;  now  if  it  were  to  rise 
twenty  feet  more,  the  new  shore  line  would  represent  the  forty 
foot  contour,  and  so  on. 

Contour  interval,  abbreviated  V.  I.,  is  the  vertical  distance 
between  adjacent  contours.  The  horizontal  distance,  abbrevi- 
ated H.  D.,  is  the  horizontal  interval  between  adjacent  contours. 
On  a  given  map,  the  V.  I.  is  constant,  but  the  H.  D.  varies  with 
the  slope  of  the  ground  represented.  Since  the  number  of 
contours  shows  directly  the  number  of  contour  intervals,  the 
difference  in  elevation  between  two  points  may  be  determined 
or  found  by  counting  the  number  of  contour  intervals  between 
them.  Thus,  if  the  V.  I.  of  a  given  map  were  20  feet  arid  there 
were  three  contour  intervals  between  two  points,  their  differ- 
ence in  elevation  would  be  60  feet. 

Contour  intervals,  in  addition  to  giving  the  difference  in 
elevation  between  two  points,  also  give  the  character  and  the 
degree  of  the  slope  between  those  points.  If  as  in  Fig.  2,  we 
show  a  cross  section  (vertical)  of  the  ground,  the  V.  I.  and 
H.  D.  will  represent  the  legs  of  a  right  triangle,  while  the  slope 
of  the  ground  will  represent  its  hypotenuse,  and  the  angle  BAC 
will  be  the  slope  angle. 


Military  Topography  and  Photography 


11 


In  this  triangle,  ABC,  it  will  be  noticed  that  the  tangent 

V.  I 

of  the  slope  angle  is  BC  divided  by  AC,  or  tan  BAG  = 

rl.  JJ. 

Since  the  V.  I.  in  this  equation  for  any  given  map  is  constant 
the  H.  D.  will  vary  inversely  with  the  tangent  of  the  slope  angle. 

8 


horizontal  Distance  (H-D) 
Fio.  2 


The  variation  in  the  tangent  of  angles  between  1°  and  20°  is 
so  closely  with  the  size  of  the  angle,  that  we  may  disregard  the 
tangent  of  the  angle  when  less  than  20°,  and  say  that  the  H.  D. 

V.I. 


varies  inversely  with  the  slope  angle,  or  BAG  r 


The 


H.  D. ' 

following  table  of  H.  D.'s  is  computed  from  the  tangent  of 
the  slope  angle  for  a  V.  I.  of  20  feet : 

Slope  Angle  The  H.  D. 

1°  "  1146  Feet 

2°  573      " 

3°  382      " 

4°  286      " 

5°  229      " 

10°  113      " 

20°  55      " 

If  we  assume  that  the  H.  D.  varies  inversely  with  the  slope 
angle  instead  of  with  its  tangent,  *the  H.  D.  for  10°  will  be 


Fio.  8 


Military  Topography  and  Photography  13 

114.6  feet,  and  for  20°  it  will  be  57.3  feet.  Comparing  these 
figures  with  those  in  the  above  table  it  will  be  seen  that  the 
differences  are  so  small  that  they  may  be  ignored  for  angles 
less  than  20° ;  contour  lines  for  an  angle  twice  as  large  will  be, 
therefore,  exactly  twice  as  close  together.  Fig.  3  shows  con- 
tours spaced  for  slopes  of  different  degrees. 

CHARACTER  OF  T&E  TERRAIN 

By  the  character  of  the  terrain  we  mean  the  conformation  of 
the  ground  and  the  growth  and  objects  on  it.  In  some  places 
the  conformation  of  the  ground  is  level  or  of  uniform  and  easy 
slopes,  in  other  places  the  ground  is  hilly  or  the  planes  cut  with 
deep  valleys,  in  other  places  the  terrain  is  that  of  bold  moun- 
tains with  steep  and  rough  slopes :  by  spacing  the  contour 
lines  so  as  to  correctly  show  the  slope  of  the  ground  at  each 
vertical  plane  of  the  earth,  the  conformation  of  the  ground 
may  be  faithfully  represented  on  a  plane  map. 

GROUND  CONFORMATION 

The  conformation  of  the  ground  may -be  shown  by  (1) 
contours,  (2)  hachures,  and  (3)  relief.  The  contour  method 
is  the  most  common  as  well  as  the  most  accurate :  contour  lines 
have  already  been  explained.  Hachures  are  short  radiating 
lines  pointing  from  higher  elevations  to  lower :  they  are  very 
difficult  and  tedious  to  make;  they  do  not  necessarily  show 
absolute  elevations  and  the  elevations  of  points  are  usually 
shown  in  small  figures  on  such  maps.  In  relief  maps  the  con- 
formation of  the  ground  is  made  to  stand  out  by  the  effects  of 
different  degrees  of  shading  of  slopes;  they  are  the  best  topo- 
graphical maps  where  very  small  scales  are  used. 

PLANES.  By  a  plane  surface  is"meant  a  level  section  of  the 
earth,  which  varies  uniformly  with  the  curvature  of  the  earth. 
If  the  level  were  perfect  there  would  be  no  contours  at  all,  but 
even  on  our  best  representations  of  planes,  such  as  the  Great 
Plains,  Mohave  Desert,  etc.,  there  are  small  slopes,  and  there 
will  always  be  the  presence  of  contours  on  any  topographical 
map.  Planes  include  what  are  known  as  prairies  or  undulating 
planes ;  i.  e.,  ground  of  small  and  easy  slopes. 


CONTOUR  MAP — U.  S.  GEOLOGICAL  SURVEY 


Military  Topography  and  Photography 


15 


SLOPES.  For  uniform  slopes,  contours  will  be  spaced  equi- 
distance  apart ;  for  convex  slopes,  the  contours  will  be  closer 
together  at  the  bottom  of  the  slope  and  farther  apart  at  the 
top ;  for  concave  slopes  the  contours  will  be  farther  apart  at 


FIG.  4 — CONVEX  SLOPE 


FIG.  5 — CONCAVE  SLOPE 


the  bottom  of  the  slope  and  closer  together  at  the  top.  Slopes 
in  old  lands  will  generally  be  found  convex  at  the  top  of  hills 
and  concave  at  the  bottom. 

WATER  SHEDS.  Water  sheds  are  high  ground,  ridges,  or 
spurs,  that  separate  different  drainage  areas.  Such  ridges, 
spurs,  or  other  high  ground,  always  have  contours  of  the  same 
elevations  on  both  sides ;  for  a  single  ridge  these  contours  close 
on  themselves,  while  for  spurs  they  will  connect  on  the  cor- 
responding contours  of  the  main  ridge. 

WATER  COURSES.  Water  courses  always  define  valleys.  On 
either  side  of  the  water  course,  there  will  always  be  found 
some  contours  of  the  same  elevations  which  always  bend  up 
stream  before  joining  each  other.  This  is  true  because  all 
valleys  become  narrower  at  their  head,  and  all  streams  have 


16  Military  Topography  and  Photography 

banks  which  are  more  or  less  high.  These  contours  must  follow 
the  valley  sides  and  stream  banks  until  they  intersect  the  usual 
surface  of  the  stream  where  the  corresponding  contours  of  the 
opposite  sides  are  connected  with  straight  lines. 

CLIFFS.  Cliffs  are  designated  as  steep  cliffs,  vertical  cliffs, 
and  overhanging  cliffs,  respectively,  according  as  to  whether 
the  slope  is  over  45°  but  less  than  90°,  of  90°  and  of  over  90°. 
When  a  slope  is  very  steep  the  contours  will  be  so  close  together 
on  the  map  that  the  intermediate  contours  must  be  omitted 
throughout  the  length  of  such  slopes,  and  only  every  fifth  or 
tenth  contour  drawn  according  to  the  degree  of  the  slope; 
when  the  ground  forms  a  wall  or  vertical  cliff,  all  the  contours 
including  the  highest  and  the  lowest  ones  for  the  cliff  will  run 
together  and  be  represented  by  one  line  throughout  the  length 
of  the  vertical  cliff;  and  for  an  overhanging  cliff,  contours  of 
lower  elevations  will  loop  under  those  of  higher  elevations 
throughout  the  length  of  the  overhanging  cliff — an  overhang- 
ing cliff  is  the  only  case  in  which  one  contour  may  cross  another. 
A  natural  slope  of  45°  is  very,  very  seldom  seen,  while  a  slope 
of  even  'only  35°  when  viewed  from  the  top  or  bottom  by  an 
untrained  eye  will  seem  to  be  from  70°  to  75°. 

HILLS  AND  DEPRESSIONS.  Hills  and  depressions  are  both 
represented  by  closed  contours.  Hills,  however,  generally  have 
their  exact  elevation  given  in  figures  within  the  highest  contour, 
while  depressions  usually  have  the  inner  edge  of  the  lowest 
contour  fringed  with  hachures. 

SADDLES  OR  COLS.  Whenever  a  ridge  or  hill  has  a  dip  in  its 
top  which  forms  two  knolls,  the  dip  between  them  is  called  a 
saddle  or  col.  Such  a  conformation  will  cause  two  closed  con- 
tours of  the  same  elevation  within  the  contour  of  the  next  lower 
elevation.  Contours  representing  all  conditions  will  be  found  in 
the  chapter  on  conventional  signs. 

GRAPHIC  REPRESENTATION 

Streams,  vegetation,  and  the  constructions  of  man  on  the 
terrain  are  represented  on  maps  by  symbols  which  are  usually 
suggestive  of  the  things  represented. 


18  Military  Topography  and  Photography 

WATER.  The  boundaries  of  oceans  and  lakes,  the  courses  of 
streams,  rivers,  and  canals,  etc.,  are  represented  by  blue  lines 
accurately  marking  their  map  positions. 

ARTIFICIAL  CONSTRUCTION.  Steam  and  electric  railroads, 
roads,  buildings,  cities,  towns,  etc.,  are  represented  by  black 
lines  which  are  suggestive  of  the  things  represented  and  mark 
their  locations. 

VEGETATION.  Trees,  grass,  orchards,  forests,  cultivated 
plants,  etc.,  are  represented  by  graphic  symbols  in  green. 

SCALES  OF  MAPS 

Since  a  map  is  a  representation  of  the  surface  of  the  earth 
according  to  some  given  scale,  each  point  or  object  on  the 
earth  must  be  shown  in  the  same  relation  on  the  map  as  it 
occupies  on  the  earth.  Thus,  if  point  A  is  five  miles  north  of 
point  B  and  ten  miles  east  of  point  C,  then  the  distance  on  the 
map  between  points  A  and  B  should  be  just  one-half  as  great 
as  the  distance  between  points  A  and  C  on  the  map,  and  points 
B  and  C  should  be  shown  in  the  same  direction  from  point  A  on 
the  map  as  they  are  on  the  ground.  The  scale  of  a  map  is 
therefore  the  ratio  between  the  represented  distances  on  the 
map  and  the  corresponding  actual  distances  on  the  ground. 
This  scale  may  be  expressed  or  represented  in  three  ways.  First, 
we  may  say  that  one  inch  on  the  map  represents  10,000  inches 
on  the  ground;  secondly,  we  may  say  that  one  inch  on  the 
map  represents-  three  miles  on  the  ground ;  and  thirdly,  we  may 
make  a  graphic  scale  for  the  maps  whose  units  or  divisions 
represent  actual  ground  distances. 

In  order  to  simplify  references  to  the  map  and  to  the  ground 
the  term,  Map  Distance,  in  this  book  will  be  understood  to  mean 
the  actual  length  on  the  map  between  two  plotted  points,  while 
the  actual  distance  on  the  ground  between  two  points  will  be 
understood  to  mean  the  Ground  Distance*  When  points  on  the 
ground  are  directly  referred  to,  they  will  be  written  in  capitals; 
as,  A,  B,  C,  etc. ;  when  plotted  points  on  the  map  are  referred 
to,  they  will  be  written  in  small  letters ;  as,  a,  b,  c,  etc. 

REPRESENTATIVE  FRACTION.  When  a  scale  is  expressed  as 
the  ratio  between  one  unit  on  the  map  and  the  number  of  like 


Military  Topography  and  Photography  19 

units  which  it  represents  on  the  ground,  it  is  written  as  a  frac- 
tion and  is  called  the  Representative  Fraction,  or  R.  F.  of  a 
map.  Thus : 

M.  D.  1   unit  on  the  map 

R'   R  =  G.  D.         ^g''     53,360  units  on  the  ground    =  =  63,360 

WORDS  AND  FIGURES.  The  scale  may  be  expressed  in  words 
and  figures;  as,  1  inch  on  map  ==  3  miles  on  ground,  or  that 
1  inch  on  map  =  100  miles  on  earth,  etc. 

GHAPHIC  SCALES.  We  may  draw  a  graphic  scale  on  the 
margin  of  a  map  whose  units  represent  actual  ground  distances. 
This  is  the  best  way  of  representing  the  scale  of  military  maps, 
for  by  its  use  the  mind  is  trained  to  think  in  ground  units  while 
looking  at  the  map :  in  the  other  methods  map  distances  have  to 
be  changed  into  ground  distances. 

Graphic  scales  are  divided  into  three  classes  depending  upon 
the  purpose  for  which  they  are  constructed  or  used:  (1) 
Reading  Scales,  (2)  Working  Scales,  and  (3)  Slope  Scales. 
This  classification  is  rather  arbitrary,  for  a  reading  scale  is 
often  used  as  a  working  scale. 

A  reading  scale  is  used  in  reading  the  map  distance  in  terms 
of  some  well-known  unit  of  lineal  measure,  as  the  inch,  centi- 
meter, etc.,  or  to  read  map  distances  in  terms  of  some  well- 
known  unit  of  land  measure,  such  as  the  yard,  mile,  kilometer, 
etc.  In  the  former  case  the  reading  scale  is  a  scale  of  equal 
parts  with  the  units  marked  in  inches '  or  centimeters ;  in  the 
latter,  the  division  or  units  on  the  scale  are  marked  in  ground 
distances,  such  as  yards,  miles,  kilometers,  etc. 

A  working  scale  is  a  scale  of  equal  parts  whose  divisions 
represent  some  convenient  number,  as  ten,  twenty-five,  or  one 
hundred  working  units,  such  as  paces,  strides,  distances  passed 
over  in  a  minute,  etc. 

A  slope  scale  is  a  scale  of  equal  parts  whose  divisions  repre- 
sent contour  intervals  for  slopes  of  different  degrees,  and 
which  is  used  to  plot  and  to  read  slopes  and  to  determine  the 
difference  in  elevation  between  points. 


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Military  Topography  and  Photography  21 

NORMAL  SYSTEM  OF  SCALES,  U.  S.  ARMY.  Maps  which  are 
made  to  larger  scales  show  less  ground  area  for  the  same  given 
map  space,  and  the  conformation  of  the  ground  on  such  maps 
can,  therefore,  be  shown  in  greater  detail  by  decreasing  the 
contour  interval  and  thereby  increasing  the  number  of  contours. 

If  the  ratio  between  the  scale  and  the  V.  I.  be  kept  constant, 
then  no  matter  what  the  scale,  the  same  degree  of  slope  will 
always  be  shown  by  the  same  spaced  contours.  We  have  seen 
that  the  H.  D.  varies  inversely  with  the  degree  of  the  slope  up 
to  20° ;  it  is  also  plain  that  the  H.  D.  for  the  same  V.  I.  and 
degree  of  slope  will  vary  with  tlje  scale  of  the  map :  if,  therefore, 
we  vary  the  V.  I.  inversely  with  the  scale  of  the  map,  we  can 
keep  the  map  distance  of  the  H.  D.  for  the  same  degree  of 
slope,  the  same  magnitude  for  maps  of  all  scales. 

Road  sketches  are  made  to  a  scale  of  3  inches  to  1  mile,  with 
a  V.  I.  of  20  feet ;  area  sketches  are  made  to  a  scale  of  6  inches 
to  1  mile,  with  a  V.  I.  of  10  feet;  War  Game  and  Fortress  Maps 
are  made  to  a  scale  of  12  inches  to  1  mile,  with  a  V.  I.  of  5  feet; 
Field  Maneuver  Maps  are  generally  made  to  a  scale  of  3  inches 
to  1  mile,  with  a  V.  I.  of  20  feet;  there  is  also  a  Strategical 
Map  which  is  made  to  a  scale  of  1  inch  to  1  mile,  with  a  V.  I. 
of  60  feet. 

From  the  following  table  it  will  be  seen  that  in  all  army 
sketches  and  maps,  the  product  of  the  M.  D.  in  inches  per  mile 
and  the  V.  I.  in  feet  is  always  60. 

M.  D.  V.  I.  Product 

(inches)  (feet) 

Road  Sketches  3          X          20  60 

Position   Sketches  6  10  60 

War  Game,  Fortress  Maps       12  5  60 

Strategical  Maps  1  60  60 

Maneuver  Maps  3  20  60 

If,  therefore,  it  is  desired  to  know  what  the  V.  I.  of  a  certain 
military  map  or  sketch  should  be,  it  may  be  easily  found  by 
dividing  60  into  the  number  of  inches  per  mile  on  the  map  or 
sketch  whose  V.  I.  is  desired. 

To  CONSTRUCT  A  READING  SCALE.  In  making  any  kind  of  a 
graphic  scale  for  a  map,  the  equal  parts  or  divisions  on  it 


22  Military  Topography  and  Photography 

should  represent  a  convenient  number  in  hundreds  of  ground 
units,  say  feet  or  yards,  so  that  these  equal  parts  or  divisions 
may  be  easily  subdivided.  For  example,  it  is  desired  to  con- 
struct a  Reading  Scale  in  yards  for  a  map  whose  scale  is,  "6 
inches  to  1  mile" ;  the  scale  to  be  about  six  inches  long. 

1st,  Find  out  how  many  yards  on  the  ground  is  represented 
by  six  inches  on  the  map ;  2nd,  Take  the  nearest  convenient 
hundred  of  yards  to  the  number  thus  found,  and  then  find  how 
many  inches  on  the  map  represent  the  selected  number  of 
yards;  3rd,  Divide  the  length  in  inches  thus  obtained  into  a 
convenient  number  of  equal  parts  and  subdivide  the  latter  into 
convenient  smaller  units — similar  to  a  measuring  ruler. 

Scale:  "6  inches  =  1  mile." 

6  inches  =  1,760  yards. 

We  shall  select  1,600  yards  as  a  convenient  number  of  yards 
for  our  Reading  Scale. 

1,760  yards  =  6  inches. 

1  yard      =  6"  ~h  1,760  =  6/1760  inches. 

1,600  yards  —  6/1760  X  1,600  =  5.06  inches. 

5.06  inches  will  therefore  be  the  length  of  our  scale. 

Measure  off  a  distance  of  5.06  inches  and  divide  this  distance 
into  four  equal  parts  as  shown  in  the  diagram  in  Fig.  6.  Then 
each  division  is  equal  to  400  yards,  and  may  be  further  sub- 
divided into  divisions  of  100,  50,  25  yards,  and  so  on. 

To  CONSTRUCT  A  SLOPE  SCALE.     In  a  previous  paragraph, 
p.  11,  it  was  seen  that  the  H.  D.  between  adjacent  contours 
for  a  slope  of  1°  and  a  V.  I.  of  20  feet,  is  1,146  feet,  and  that 
for  all  angles  up  to  20°,  the  H.  D.  can  for  all  practical  pur- 
poses be  taken  to  vary  inversely  with  the  degree  of  the  slope. 
For  a  map  whose  scale  is  3  inches  to  1  mile  : 
3     inches  =  5280  feet, 
1      inch       =1760   feet, 

1146 

1146  feet  =          -  inches  =  .65  +  inches. 
IToU 

It  was  also  seen  that  the  V.  I.  was  made  to  vary  inversely  with 
the  scale  of  the  map.  Thus  the  V.  I.  for  a  scale  of  6  inches  to 
1  mile  is  10  feet.  Therefore  the  H.  D.  between  adjacent  con- 


Military  Topography  and  Photography  23 

tours  for  a  slope  of  1°  and  a  V.  I.  of  10  feet  is  573  feet.   For  a 
map  whose  scale  is  6  inches  to  1  mile,  therefore  : 

6  inches  —  5280  feet, 

1   inch  880  feet, 


573  feet  —  1   inches  =  .65  +  inches. 

880 

Therefore,  for  all  maps  made  to  the  normal  system  of  scales, 
U.  S.  A.,  slopes  of  the  same  degree  will  always  be  shown  by  con- 
tours spaced  the  same  distance  apart,  no  matter  what  the  par- 
ticular scale  is,  and  the  same  slope  scale  can  be  used  on  any  of 
these  maps. 

Now  construct  a  scale  as  shown  in  Fig.  7.  Instead  of  measur- 
ing .65  of  an  inch  off  directly  for  the  H.  D.  of  a  1°  slope,  it  is 
more  accurate  to  measure  off  (5  X  .65  =)  3.25  inches  and  divide 
this  length  into  five  equal  parts.  This  distance,  .65  of  an  inch, 
divided  into  two  gives  the  H.  D.  for  a  2°  slope;  into  four,  a  4° 
slope;  into  eight,  an  8°  slope:  similarly,  into  three,  a  3°  slope; 
into  six,  a  6°  slope;  into  twelve,  a  12°  slope:  and  similarly,  into 
one  and  one-fourth,  a  l^0  slope;  into  two  and  one-half,  a  2^/2° 
slope;  into  five,  a  5°  slope;  into  ten,  a  10°  slope,  and  so  on. 

The  H.  D.  for  a  slope  of  1%°  is  (.65  -=-11/4  =)  -52  of  an 
inch.  To  get  divisions  of  this  magnitude  measure  off  a  line  2.6 
inches  long  and  divide  it  into  five  equal  parts.  The  M.  D.  of 
the  H.  D.  for  any  slope  up  to  20°  can  be  found  by  dividing  .65 
inches  by  the  degree  of  the  slope. 

ORIENTATION  OF  MAPS.  By  the  "Orientation  of  a  Map"  is 
meant  turning  the  map  in  such  a  position  in  a  horizontal 
plane  so  that  the  directions  on  the  map  coincide  with  those  on 
the  ground.  The  map  may  be  placed  in  a  horizontal  plane  by 
spreading  it  out  on  a  table,  or  laying  it  flat  on  level  ground,  or 
holding  it  level  in  the  hand.  It  might  be  well  to  mention  a  few 
facts  of  elementary  geography,  which,  if  kept  in  the  mind,  will 
prevent  confusions  of  directions.  If  a  man  stands  with  his  face 
towards  the  north,  then  his  back  will  be  towards  the  south,  his 
right  hand  towards  the  east,  and  his  left  hand  towards  the  west. 
In  whatever  way  one  faces,  the  top  of  the  map  is  always  north, 
the  bottom  south,  the  right-hand  edge  east,  and  the  left-hand 
edge  west.  Since  the  top  of  a  map  is  always  north,  it  is  there- 


24  Military  Topography  and  Photography 

fore  only  necessary  to  know  the  north  on  the  ground  in  order  to 
bring  the  directions  on  the. map  into  coincidence  with  the  direc- 
tions on  the  ground  or  to  "orient  the  map,"  as  it  is  commonly 
called.  We  may,  therefore,  orient  a  map  with  the  aid  of  any 
method  of  determining  directions  on  the  ground. 

(1)  Orientation  by  Compass:     When  the  compass  is  used 
to  determine  the  approximately  true  north,  the  magnetic  declina- 
tion must  be  known   and   allowed   for.      Most   maps   have   an 
arrow  pointing  towards  the  true  north ;  this  line  is  parallel  with 
the  border  line  on  the  side  of  map.    In  addition  to  the  true  north 
arrow,  there  is  generally  a  secondary  arrow  making  an  angle 
with  the  true  north  arrow,  which  angle  represents  the  magnetic 
declination  for  the  locus  of  the  map.    If  the  secondary,  or  mag- 
netic arrow  on  the  map  is  made  to  point  in  the  same  direction 
as  the  needle  of  a  compass,  then  the  true  north  arrow  will  point 
towards  the  true  north,  and  the  map  will  be  correctly  oriented. 

(2)  Orientation  by  the  Sun:     In  the  northern  hemisphere 
generally,  and  always  north  of  the  Tropic  of  Cancer,  the  sun  at 
noon  is  directly  south.     If,  therefore,  the  south  of  a  map  is 
directed  towards  the  sun  at  noon  it  will  be  correctly  oriented. 
It  is  not  always  possible,  however,  to  know  just  when  it  is 
exactly  noon,  but  by  using  the  following  method  an  approximate 
true  north  and  south  line  may  be  determined.     See  Fig.  8.    Take 


Fia.  8 


Military  Topography  and  Photography  25 

a  straight  stick  three  or  four  feet  long  and  stick  it  into  the 
ground  so  that  it  leans  towards  the  north  as  nearly  as  it  can  be 
estimated.  Support  this  stick  by  means  of  two  crossed  sticks 
which  are  securely  stuck  into  the  ground,  and  securely  fasten 
the  leaning  stick  to  them  by  tying  them  together  at  their  com- 
mon junction  with  a  small  rope.  Suspend  from  the  free  end  of 
the  leaning  stick  a  plumb  bob  A  by  means  of  a  cord  of  such 
length  that  the  plumb  bob  is  just  free  from  the  ground;  observe 
at  about  thirty  minutes  before  noon  where  the  shadow  from  the 
free  end  of  the  leaning  pole  falls,  and  with  the  distance  from  this 
shadow  to  point  A  as  a  radius  and  point  A  as  a  center  describe 
an  arc  of  a  circle  on  the  ground,  which  ground  should  be  level ; 
observe  and  mark  the  point  where  the  shadow  from  the  free  end 
of  the  leaning  pole  crosses  the  described  arc  after  noon ;  bisect 
the  arc  connecting  the  two  points  C  and  D,  where  the  shadow 
intercepts  the  described  arc;  join  this  point  E  with  A  and  the 
line  AE  will  be  a  true  north  and  south  line. 

(3)  Orientation  by  North  Star:  The  North  Star,  or 
Polaris,  with  only  a  very  small  daily  cyclic  variation,  is  in  the 
true  north  direction;  therefore,  by  aligning  two  points  on  the 
earth  with  it  (after  night  when  it  can  be  easily  seen)  an  ap- 
proximate true  north  and  south  line  can  be  obtained.  The 
method  usually  employed  to  do  this  is  as  follows :  A  plumb  bob 
and  line  are  suspended  from  an  object,  such  as  a  tree  limb,  which 
is  8  or  10  feet  from  the  ground,  so  that  the  plumb  bob  is  just 
free  from  the  ground — the  plumb  bob  may  be  allowed  to  swing 
in  a  bucket  full  of  water,  especially  if  it  is  windy,  in  order  to 
dampen  its  oscillations;  by  sighting  on  the  plumb  line  and 
Polaris,  one  stake  is  driven  about  10  feet  directly  south  of  the 
plumb  line;  and,  by  sighting  on  this  stake  and  the  plumb  line 
another  is  driven  the  same  distance  directly  to  the  north. 
These  two  stakes  will  then  be  in  an  approximately  true  north 
and  south  line.  By  the  following  modification  of  this  method 
a  much  more  accurate  north  and  south  line  may  be  obtained. 

Proceed  as  above,  and  after  having  established  the  two  stakes, 
accurately  determine  on  the  top  of  the  first  stake  by  means  of 
a  small  nail  or  movable  peep  sighting  vane  the  exact  point  where 
the  line  of  sight  from  Polaris  to  the  plumb  line  strikes  the  stake ; 


Military  Topography  and  Photography 


27 


then  by  means  of  <'i  .small  nail  mark  the  point  on  top  of  the 
second  stake  where  the  line  of  sight  from  the  peep  hole  to  the 
plumb  line  strikes  the  second  stake;  observe  the  clock  reading  of 
the  "Great  Dipper"  (Ursa  Major)  or  "Cassiopeia  Chair,"  and 
from  the  table  on  page  247  find  the  azimuth  of  Polaris  for  that 
time.  The  line  determined  by  the  points  on  the  two  stakes,  plus 
or  minus  the  azimuth  of  Polaris  for  that  time,  is  a  true  north 
and  south  line.  The  plumb  line  should  be  4  or  y1^  of  an  inch 
in  diameter  and  should  be  chalked  white — both  the  plumb  line 
and  the  small  nail  marker  on  the  far  stake  should  be  illuminated 
by  a  lamp  or  lantern,  which  light  should  be  shaded  towards  the 
first  or  observing  stake. 


FIG.  10 


(4)  Orientation  by  Watch:    If  the  hour  hand  of  a  watch  is 
pointed  directly  towards  the  sun,  then  the  point  on  the  dial  half 
way  between  the  hour  hand  and  XII  o'clock  will  point  towards 
the  true  south. 

(5)  Orientation  by  Comparisons:    Generally  a  ridge,  road, 
railroad,  fence,  or  other  object  on  the  map  may  be  recognized 


28  Military  Topography  and  Photography 

on  the  ground.  In  such  cases  it  is  only  necessary  to  bring  the 
line  of  direction  of  such  ridge,  road,  railroad,  or  other  object  on 
the  map  into  coincidence  with  its  line  of  direction  on  the  ground 
for  the  corresponding  point  and  the  map  will  be  correctly 
oriented.  This  furnishes  a  rapid  and  fairly  accurate  method 
for  orienting  maps  and  is  much  used. 

LOCATION  OF  MAP  POSITION.  In  order  to  read  a  map  it  is 
first  necessary  to  orient  the  map,  and  then  to  locate  one's  ground 
position  on  the  map.  Often  one  is  near  a  distinctive  object  on 
the  terrain  which  is  plainly  plotted  on  the  map  by  means  of  some 
appropriate  conventional  sign ;  one  is  then  able  to  locate  his 
map  position  at  once.  More  often,  however,  one  is  not  so  fortu- 
nate; it  is  then  necessary  to  resort  to  certain  geometrical  but 
simple  aids  in  locating  one's  self.  The  geometrical  aids  are 
familiarly  known  as  "Resection  Problems." 

(1)  By  Two  Plotted  Points:    Two  points,  A  and  B,  on  the 
terrain  are  plotted  on  the  map  as,  a  and  b  (the  points,  A  and  B, 
may  be  conspicuous  knolls,  churches,  or  other  easily  recognized 
objects  that  are  plotted  on  the  map)  ;  the  map  is  accurately 
oriented  by  means  of  one  of  the  methods  described  above;  a 
straight-edge  ruler  is  then  placed  on  the  map  so  that  it  just 
touches  point  a,  and  with  a  as  a  center  the  ruler's  edge  is  re- 
volved about  a  to  such  a  position  that  by  sighting  along  its  edge 
the  point  A  on  the  terrain  may  be  seen  ;  a  light  pencil  ray  is  then 
drawn  along  the  edge  of  the  ruler  towards  the  observer.     Simi- 
larly the  ruler  is  aligned  on  b  and  B,  and  a  ray  from  b  is  drawn 
towards  £he  observer.     The  point  where  the  two  rays  intersect 
will  be  the  map  position  required.     By  looking  at  the  diagram, 
Fig.  11  (a),  no  further  explanation  will  be  needed.     The  large 
square  is  supposed  to  be  the  actual  ground  while  the  smaller 
square  is  supposed  to  be  the  map. 

(2)  By  Three  Plotted  Points:     Often  the  magnetic  decli- 
nation of  a  place  is  not  known,  or  the  local  magnetic  attraction 
is  so  great  as  to  make  the  compass  unreliable,  and  it  is  not  pos- 
sible to  orient  the  map  by  any  of  the  other  methods  explained 
above ;  the  map  may  then  be  both  oriented  and  one's  position  de- 
termined by  means  of  three  ^plotted  points.     Three  point  resec- 


CP 


I    s 

I    \ 

I        \ 


V          '  ' 

A  V  ' 

\  \  I 

>   •      / 


(b) 


Fio.  11 


30  Military  Topography  and  Photography 

tion  is  usually  applied  either  directly  on  the  map  by  trials,  or 
indirectly  by  means  of  tracing  paper.     Fig.  11  (b). 

(a)  By  Trials:     The  map  is  first  oriented  as  accurately 
as  it  can  be  by  estimation;  sight  rays  by  means  of  a  straight 
edge  ruler  are  then  taken  on  a  and  A,  b  and  B,  and  c  and  C, 
respectively,  and  lines  drawn  towards  the  observer  each  time. 
If  the  three  lines  intersect  in  the  same  point,  the  map  is  correctly 
oriented ;  if  not,  the  map  is  slightly  revolved  to  the  right  or  left 
according  to  the  rules  on  pages  67-73,  and  rays  are  drawn  until 
the  three  lines  do  intersect  in  a  point.     The  "Three-Point  Prob- 
lem" may  be  included  as  a  method  of  orienting  maps. 

(b)  By  Tracing  Paper:    A  point  o  is  conveniently  plot- 
ted on  a  piece  of  tracing  paper ;  with  the  tracing  paper  station- 
ary, sight  rays  with  a  straight-edge  ruler  are  taken  from  point 
o  on  objects  A,  B,  and  C,  light  pencil  rays  being  drawn  each 
time  to  represent  them.     The  tracing  paper  is  then  applied  to 
the  map  and  so  adjusted  that  the  pencil  rays  exactly  coincide 
with  the  respective  plotted  points  a,  b,  and  c  at  the  same  time; 
point  o  on  the  tracing  paper  is  then  exactly  over  the  map  posi- 
tion required. 

(3)  By  "Ranging  In" :     Often  one  is  on  some  road,  rail- 
road, ridge,  or  other  like  object  which  he  recognizes  on  the  map, 
but  he  cannot  tell  just  at  what  point  along  that  road,  railroad, 
or  ridge  he  is,  and  he  is  able  to  see  or  recognize  only  one  plotted 
point  outside  of  that  line.     He  may  then  locate  his  position  by 
what  is  known  as  "ranging  in."     The  map  is  first  accurately 
oriented,  a  sight  ray  is  then  taken  from  a  on  A,  the  known 
plotted  object,  with  a  straight-edge  ruler,  and  a  light  pencil 
ray  drawn  to  represent  it.     Where  this  ray  intersects  the  road, 
railroad,  ridge,  or  other  like  object,  is  the  map  position  required. 

(4)  By  "Lining  In":     Sometimes  one  finds  himself  in  a 
rather  low  piece  of  ground  or  a  wooded  place,  where  he  can  see 
only  one  plotted  point,  A,  on  the  terrain,  but  he  is  able  to  see 
other  ground  which  he  is  unable  to  recognize  on  the  map.     In 
such  positions  he  can  often  locate  his  map  position  by  what  is 
commonly  called  "lining  in."     The  map  is  accurately  oriented 
and  a  sight  ray  from  a  on  A  is  ta.ken  with  a  straight-edge  ruler 
and  a  pencil  ray  is  drawn  on  the  map  to  represent  it.     He  then 


Military  Topography  and  Photography  31 

goes  to  the  other  ground  lie  could  not  recognize,  is  able  to  locate 
a  point  on  it  by  resection  on  other  visible  plotted  points,  and 
takes  a  ray  on  the  initial  point.  Where  this  ray  intersects  the 
first  ray  drawn,  is  the  map  position  required. 

In  locating  one's  self  by  resection,  the  angle  formed  by  the 
intersecting  lines  should  not  be  less  than  30°  nor  more  than 
120°;  in  three-point  problems  the  angle  formed  by  the  exterior 
intersecting  lines  is  to  be  considered.  In  addition  to  the  above 
general  methods  of  resection,  there  are  certain  special  applica- 
tions or  modifications  which  are  taken  up  under  "Plane  Table 
Operations,"  page  61,  which  may  also  be  used  for  map  orienta- 
tion and  location. 

VISIBILITY 

OF  POINTS.  Two  points  on  the  terrain  are  of  course  visible 
one  from  the  other  unless  there  is  some  object  between  them 
which  obstructs  the  line  of  sight.  Such  obstructing  objects  may 
be  hills,  buildings,  woods,  or  any  other  like  feature  of  the  terrain. 
We  all  know  that  points  across  a  narrow  valley  are  visible  to 
points  on  the  other  side  of  that  valley,  while  points  on  one  side 
of  a  hill  are  invisible  to  points  on  the  other  side  of  that  hill ;  but 
where  the  terrain  between  two  points  is  of  varied  conformation, 
it  is  generally  very  difficult  to  tell  whether  the  points  are  inter- 
visible  unless  one  of  the  points  is  actually  occupied  or  the 
terrain  is  graphically  represented.  Since  the  actual  conforma- 
tion of  the  ground  is  represented  on  a  topographical  map  by 
contours,  the  visibility  of  points  to  each  other  may  be  easily 
determined  by  observing  the  spacing  and  elevation  of  contours 
between  them. 

The  conformation  of  the  terrain  between  two  points  may  be 
considered  in  two  general  classes — continuous  and  interrupted 
slopes.  The  slope  between  two  points  is  said  to  be  continuous 
when  it  is  uniform,  and  interrupted  when  it  is  of  several  slopes 
of  different  degree.  There  are  three  kinds  of  continuous  slopes, 
level,  convex,  and  concave.  When  the  slope  between  two  points 
is  uniform  and  the  ground  open,  those  points  are  intervisible ; 
when  the  slope  is  concave,  they  are  also  intervisible;  but  when 
the  slope  between  two  points  is  convex,  they  are  invisible  to  each 
other.  In  general  then  for  continuous  slopes:  When  the  con- 


RELIEF  MAP 

From  Morton's  Elementary  Geography.     By  permission  of  American   Book  Company, 

Publishers. 


Military  Topography  and  Photography  ,    33 

i 

tours  near  the  higher  of  two  points  are  farther  apart  than  they 
are  near  the  lower  of  those  two  powts,  the  points  are  not  inter- 
visible,  and  vice  versa. 


4.^^ 


R 


« 


*SLIDE  RULE 

When  the  ground  between  two  points  is  composed  of  several 
slopes  of  different  degree,  it  is  much  more  difficult  to  tell  from  a 
map  as  to  whether  or  not  theyjare  intervisible.  It  is  only  neces- 
sary, however,  to  determine  whether  there  are  any  objects 
obstructing  the  line  of  sight.  Such  objects  must  of  course  be  in 
the  same  horizontal  line  as  the  two  points  and  above  the  line  of 
sight  between  them,  so  it  will  only  be  necessary  to  tell  whether 
any  object  in  the  same  vertical  plane  as  the  two  points  is  above 
the  gradient  of  those  two  points.  From  Fig.  2,  it  may  be  seen 
that  any  two  points  of  different  elevations  may  define  a  vertical 
right  triangle,  in  which  the  slope  distance  between  the  two  points 
is  the  hypotenuse,  and  their  vertical  and  horizontal  distances  the 
legs  of  that  triangle.  It  will  be  remembered  (p.  12)  that  the 
gradient  of  a  slope  is  the  ratio  between  its  vertical  projection 
to  its  horizontal  projection,  fractionally  expressed.  Thus,  in 
Fig.  2,  the  gradient  of  slope  AB  is  BC/AC,  in  which  both  BC 
and  AC  must  be  expressed  in  the  same  sized  units. 

From  Fig.  12,  it  may  be  seen  that  if  an  object  D'  of  the  ter- 
rain actually  obstructs  the  line  of  sight  between  any  two  points, 
then  a  second  right  triangle  ADE  is  formed  by  a  vertical  line 
dropped  from  the  obstructing  point,  in  which  triangles  the  angle 
BAG  is  common,  and  angle  ABC  equals  angle  ABE.  Therefore, 
since  the  homologous  sides  of  similar  triangles  are  proportional, 
AC:AE::BC:DE,  or  ACXDE  =  AEXBC,  but  ACXD'E> 
AEXBC,  from  which  we  may  formulate  the  rule  that,  //  the 
product  of  the  horizontal  distance  between  the  two  points  times 
the  difference  in  elevation  between  the  lower  point  and  the  ob- 
structing point  is  greater  than  tlfie  product  of  the  horizontal 
distance  between  the  lower  point  .and  obstructing  point  times  the 

*  Courtesy  of  W.  &  L.  E.  Gurley. 


*J4<  •    Military  Topography  and  Photography 

difference  in  elevation  between  the   two  points,   then   the   two 
points  are  not  intervisible ;  if  less,  they  are  inter  visible.     Point 


FIG.  12 

D'  is  taken  as  the  probable  obstructing  point.     Similarly,  from 
Fig.  12: 

DE  :BC  ::AE  :  AC,  and 

BC/AC  =  DE/AE  (Similar  triangles),  but 

D'E/AE  is  greater  than  DE/AE. 

Therefore,  D'E/AE  is  greater  than  BC/AC  (D'  is  an  ob- 
structing point). 

We  may  therefore  formulate  the  following  rule :  //  the  gradi- 
ent of  the  two  points  whose  intervisibility  is  sought  be  greater 
than  the  gradient  of  the  lower  of  those  two  points  and  the  prob- 
able obstructing  point,  they  are  intervisible;  if  less,  they  are 
not  intervisible. 

It  will  be  readily  seen  by  inspecting  a  topographical  map  as 
to  whether  there  is  any  object  between  two  points  which  is  likely 
to  obstruct  the  line  of  sight  between  them :  with  sufficient  prac- 
tice one  should  also  be  able  to  tell  by  sight  as  to  whether  such 
object  is  an  actual  obstructing  point  within  the  limits  of  the 
probable  accuracy  of  the  map. 

To  determine  visibility  problems,  therefore,  it  is  only  neces- 
sary to  determine  the  horizontal  and  vertical  projections  of 
slopes  and  find  the  product  of  or  quotient  between  such  pro- 
jections. The  horizontal  .projection  of  the  slope  between  two 
points,  i.  e.,  their  map  distance  apart,  may  be  measured  with  a 


FIG.  13— PROFILE  OF  LINE  AB 


36 


Military  Topography  and  Photography 


Reading  Scale  in  terms  of  ground  units,  or  with  a  rule  of  equal 
parts  in  terms  of  inches  or  centimeters :  since  contours  give  the 
elevation  of  points  directly  the  vertical  projection  of  the  slope 
between  two  points  is  equal  to  the  difference  in  elevations  be- 
tween those  points.  It  matters  not  whether  the  horizontal  pro- 
jection be  expressed  in  inches  or  centimeters  on  the  maps,  or 
yards  or  meters  on  the  ground;  the  vertical  projection  will 
always  be  expressed  in  feet  or  meters.  The  proportion  between 
these  projections  may  be  solved  on  paper,  with  a  slide  rule,  or 
in  the  head.  Mere  inspection  of  the  map  after  sufficient  practice 
will  usually  suffice. 

Visibility  of  points  in  the  same  vertical  plane  may  be  graphi- 
cally shown  by  using  the  lines  of  a  common  ruled  sheet  of  paper 
as  shown  in  Fig.  14,  or  by  projecting  the  same  in  the  form  of 
a  profile  as  shown  in  Fig.  13. 


Fio.  14 


Military  Topography  and  Photography 


37 


Fio.  15 


VISIBILITY  OF  AREAS.  From  Fig.  15  it  will  be  seen  that 
points  B  and  C  are  the  limiting  points  of  visibility  of  the  line 
or  distance  BC  with  respect  to  point  A.  If  a  sufficient  number 
of  such  limiting  points  of  visibility  of  different  lines  or  distances 
be  determined  with  respect  to  point  A,  such  points  will  also  be 
the  limiting  points  of  areas  visible  and  invisible  to  point  A. 
All  points  within  such  areas  will  of  course  be  visible  or  invisible 
to  point  A.  The  shaded  portions  of  the  map  in  Fig.  15,  show 
areas  which  are  invisible  to  point  A,  while  the  unshaded  portions 
show  the  visible  areas  with  respect  to  point  A. 


38  Military  Topography  and  Photography 

VISIBILITY  PROBLEMS.  In  order  to  secure  ease  and  rapidity 
in  the  solution  of  visibility  problems,  the  student  should  practice 
much  the  determination  of  intervisibility  of  points  between 
which  points  the  terrain  is  of  different  slopes,  using  any  good 
contour  maps. 

MAP  INTERPRETATION 

After  one  has  first  studied  the  subject  of  Map  Reading,  he  is 
very  likely  to  fall  into  the  error  of  thinking  that  he  can  apply 
the  foregoing  rules  of  map  reading  mechanically,  or  perhaps 
what  would  be  a  more  exact  statement,  his  knowledge  of  map 
reading  is  confined  to  the  mechanical  rules  he  has  learnt  from 
the  book.  It  should  be  remembered,  however,  that  a  map  is  a 
representation  and  not  an  exact  reproduction  of  the  terrain ;  to 
reproduce  all  the  details  of  a  terrain  several  miles  square  upon 
the  sheet  of  paper  several  inches  square  is  beyond  the  realm  of 
practicability  at  least.  A  topographical  map  is  much  like  a 
piece  of  literature  in  which  no  author  can  express  complete 
thought  by  the  use  of  words  alone  but  by  appealing  to  the  imagi- 
nation, to  the  intellect,  by  the  use  of  subtle  suggestions,  by  the 
choice  use  of  adjectives  and  limiting  and  modifying  phrases,  the 
mind  of  the  reader  is  led  and  directed  through  the  same  channels 
of  thought  and  reasoning  as  the  author's.  Analogous  to  "liter- 
ary interpretation,"  therefore,  we  have  "map  interpretation," 
and  he  who  has  no  knowledge  of  the  terrain  and  of  the  limits  and 
possibilities  of  topographic  sketching  is  like  a  person  who  knows 
how  to  pronounce  words  but  has  no  conception  of  their  meaning. 
No  one  can  thoroughly  interpret  a  topographic  map  who  has 
not  had  experience  as  a  sketcher.  This  does  not  mean  that  much 
benefit  cannot  be  gotten  from  a  mere  study  of  maps,  but  that  all 
officers  in  the  military  profession  should  have  had  experience 
as  topographic  sketchers. 

OF  ALL  MAPS.  In  the  interpretation  of  any  map  due  allow- 
ance should  first  be  made  for  the  methods  employed  in  its  con- 
struction, the  condition  under  which  the  sketcher  worked,  and 
the  skill  of  the  sketcher  who  made  it.  Topographic  surveys 
using  accurate  control,  such  as,  geodetic  triangulation,  control 
traverses,  etc. ;  sketches  made  with  a  base  line  measured  by 


Military  Topography  and  Photography  39 

pacing,  or  perhaps  by  estimation;  the  proximity  of  hostile 
forces ;  experienced  or  inexperienced  sketchers — all  these  factors 
present  considerations  which  the  reader  must  know  how  to 
allow  for. 

OF  ROADS.  Of  all  the  features  the  roads  will  be  the  most 
accurate.  This  is  due  to  their  ease  of  access  and  of  locating  them. 
It  is  but  human  for  the  sketcher  to  do  his  best  at  those  places 
which  are  likely  to  be  visited  most  by  those  who  are  to  use  the 
map.  Traverses  will  be  run  over  roads  or  at  least  the  more  im- 
portant ones  as  a  framework  of  control  for  the  adjacent  sketch- 
ing, or  road  crossings  and  changes  in  direction  of  roads  will  be 
determined  by  other  accurate  geometric  methods,  such  as,  re- 
section, intersection,  etc.  For  maps  executed  in  the  field,  it  may 
be  safely  assumed  that  horizontal  points  will  be  plotted  within 
1/50  of  an  inch  of  the  determined  map  position ;  points  deter- 
mined by  plane  table  methods  for  average  sized  maps  will  be 
plotted  within  50  feet  of  their  true  map  position  as  determined ; 
while  in  sketches,  points  determined  by  estimation  will  vary 
according  to  the  distance  from  the  point  or  points  from  which 
they  are  estimated — distances  estimated  will  vary  from  0  to  10 
per  cent,  or  perhaps  greater,  of  their  true  value,  according  to 
the  skill  of  the  sketcher. 

OF  STREAMS.  The  location  of  streams  will  be  the  next  to 
roads  (including  railroads,  etc.)  in  their  accuracy.  The  courses 
of  important  streams  will  usually  be  determined  by  traverses, 
or  important  points  on  them  determined  by  other  accurate 
geometric  methods. 

OF  THE  CONFORMATION  OF  THE  GROUND.  It  is  of  course 
apparent  that  it  is  impracticable — too  costly  and  too  laborious, 
to  determine  a  sufficient  number  of  points  on  the  terrain  so  as  to 
make  the  horizontal  location  of  contours  so  accurate  that  their 
error  would  always  be  within  the  limits  of  plotting — say  l/50th 
of  an  inch.  The  sketcher  must  therefore  determine  only  a  suffi- 
cient number  of  important  or  critical  points  to  control  his  work, 
and  then  plot  the  contours  by  eye  so  as  to  represent  the  slopes 
and  conformation  of  the  ground  as  they  appear  to  him.  Even 
with  the  same  control  the  most  accurate  sketchers  will  vary 
slightly  in  the  spacing  of  contours  by  estimation,  but  all  will 


40  Military  Topography  and  Photography 

faithfully  represent  the  conformation  of  the  ground.  In  the 
readings  of  the  contours  of  a  map,  the  conformation  of  the 
ground  must  be  interpreted  in  the  light  of  a  knowledge  of 
ground  forms.  Military  problems  depend  greatly  upon  the 
tactical  possibilities  of  the  terrain,  and  when  such  problems  are 
solved  from  maps,  such  knowledge  is  essential  to  proper  solu- 
tions. 

OF  FORESTS.  The  presence  of  dense  forests  on  the  ground 
renders  the  details  of  its  conformation  of  very  little  importance, 
but  which  in  open  ground  would  be  of  great  military  impor- 
tance. The  presence  of  forests  renders  the  determination  and 
plotting  of  details  of  the  ground  impossible;  the  sketcher  does 
not  enter  forests  in  search  of  such  details  and  he  would  not  find 
them  if  he  did.  The  contours  running  through  woods  should, 
therefore,  be  interpreted  as  representing  only  the  broad  confor- 
mation of  the  ground. 

OF  CONVENTIONAL  SIGNS.  Conventional  signs  like  print 
must  be  large  enough  to  be  seen  and  read.  We  cannot  therefore 
draw  them  to  scale,  but  must  represent  them  by  symbols  of 
convenient  size  without  regard  to  the  scale  of  the  map.  Thus  the 
size  of  a  palm  tree  as  shown  in  the  official  book  of  conventional 
signs,  U.  S.  Army,  when  shown  on  a  three  inch-to-one-mile  map, 
is  almost  300  feet  high  and  200  feet  in  diameter;  an  orchard- 
tree  symbol  is  125  feet  in  diameter;  a  single  cannon  is  about 
175  feet  long,  while  a  sentinel  is  65  feet  in  diameter.  As  a  rule 
only  a  few  symbols  are  shown  to  represent  a  certain  vegetation 
area;  or  the  location  of  a  single  tree  in  an  open  area;  or  a 
sentry,  picket,  or  support  in  an  outpost  position,  and  so  on. 
The  solid  built  sections  in  cities  will  be  shown  by  solid  blocks ; 
dwelling  houses  in  residential  districts  will  be  shown  only  con- 
ventionally by  the  proper  symbols — not  the  exact  number;  all 
farm  dwelling  houses  are  shown  but  not  the  attached  barns  and 
other  outhouses;  detached  barns  and  other  buildings,  however, 
will  be  shown. 

ON  THE  SYSTEMATIC  READING  OF  MAPS.  To  read  a  map  it 
is  first  necessary  to  orient  the  map  and  then  to  locate  one's  posi- 
tion on  it.  A  sufficient  number  of  prominent  points  should  next 


Military  Topography  and  Photography  41 

be  located  both  on  the  ground  and  the  map  so  as  to  keep  one's 
idea  of  directions  constantly  true. 

Military  men  have  to  read  maps  under  two  different  condi- 
tions— 1st,  in  conjunction  with  the  terrain,  and  2nd,  from  the 
map  alone.  In  garrison  school  work,  map  problems  will  usually 
be  given  on  maps  whose  terrain  many  of  the  officers  have  never 
seen;  here  the  terrain  must  exist  in  the  imagination  of  the 
student  and  a  knowledge  of  ground  forms  is  essential  to  a  true 
picture.  In  maneuvers  and  campaigns,  maps  of  the  theater  of 
operations  will  usually  be  furnished,  so  that  such  maps  may  be 
read  in  conjunction  with  the  terrain. 

A  map  cannot  be  read  at  a  glance,  but  must  be  studied  inch 
by  inch.  Preparatory  to  a  detailed  study  of  the  map,  however, 
the  map  should  be  looked  at  in  a  general  way  in  order  to  get  a 
general  idea  of  the  ground.  The  courses  of  streams  should  first 
be  noted,  this  will  at  once  give  the  sections  of  lowest  and 
of  highest  elevations,  the  watersheds  and  valleys ;  it  will  show 
whether  the  stream  beds  are  deeply  cut  or  not  and  give  a 
general  idea  as  to  the  conformation  of  the  ground. 

If  the  map  is  then  held  straight  out  in  front  of  the  eye  at  a 
convenient  distance,  the  hills,  ridges  and  slopes  should  appear 
to  stand  out  in  relief,  as  if  the  map  were  a  model  of  the  ground 
itself.  If  an  even  hundred-foot  contour  which  is  common  to  the 
whole  map  be  colored,  say  with  red  crayon,  this  effect  will  be 
even  more  vivid  and  its  use  is  recommended.  The  streams,  of 
course,  should  appear  lower  at  their  exits  and  higher  at  their 
sources. 

After  gaining  a  general  idea  of  the -terrain,  the  map  should 
be  studied  in  detail;  the  location  and  names  of  cities,  towns, 
villages,  farmhouses,  churches,  schoolhouses,  etc.,  should  be 
noted ;  the  location,  name  and  the  direction  and  places  to  which 
they  lead,  should  be  noted  of  all  roads,  railroads,  electric-roads, 
telegraph  and  telephone  lines,  etc. ;  the  general  visibility  of 
areas  to  prominent  points  in  the  terrain  should  be  determined ; 
and  the  character  and  features  of  the  terrain  on  the  whole  map 
should  be  systematically  noted  inch  by  inch. 

With  proper  study  and  experience  in  map  reading  and  sketch- 
ing will  come  ease  and  thoroughness  in  their  interpretation.  A 


42  Military  Topography  and  Photograpliy 

map  should  be  so  thoroughly  studied  during  maneuvers  and  cam- 
paign that  ground  never  seen  before  can  be  recognized  by 
memory  from  the  map,  while  points  of  reference  which  might  be 
used  in  field  orders  should  already  be  known  by  those  who  may 
have  to  carry  into  execution  suclf  orders. 

ON  CONTOURS  IN  GENERAL.  The  following  rules  in  regard 
to  contours  should  always  be  borne  in  mind : 

a.  All  points  on  a  contour  line  have  the  same  elevation. 

b.  Where  the  contours  are  evenly  spaced  the  slope  is  uni- 
form. 

c.  If  the  contours  are  closer  together  at  the  bottom  part 
of  a  slope,  the  slope  is  convex;  if  closer  together  at  the  top, 
the  slope  is  concave. 

d.  All  the  contours  of  a  vertical  cliff  form  a  single  line. 

e.  Every  contour  either  closes  on  itself  or  runs  clear  across 
the  map. 

f.  On   watersheds   and    spurs    of   hills,   the    lower    contours 
bulge  outwards  from  the  higher  contours;  in  water  courses  or 
ravines,  the  lower  contours  bend  inwards  towards  the  higher. 

g.  A  series  of  concentric  (closed)  contours  represents  a  hill. 
A  closed  contour  containing  within  it  no  other  contour  generally 
represents  a  hill;  a  depression  would  be  occupied  by  a  pond  or 
lake. 

h.  All  contours  adjacent  to  a  stream  whose  elevations  are 
lower  than  the  source  of  a  stream  must  cross  that  stream  some- 
where below  its  source. 

i.  A  contour  crossing  a  stream  always  turns  upstream  in 
crossing,  forming  an  inverted  V. 


CHAPTER  II 

TOPOGRAPHICAL  SURVEYING 

GENERAL  REMARKS.  Topographical  surveying  consists  of 
two  distinct  operations  carried  on  chiefly  in  conjunction,  the 
control  work  and  the  sketching.  The  former  is  geometric  and 
requires  only  a  knowledge  of  and  ability  to  use  surveying  in- 
struments of  precision,  while  the  latter  is  artistic  and  requires 
much  practice  and  a  proper  conception  of  ground  forms  and 
the  ability  to  see  them  in  their  proper  relation  to  one  another 
in  order  to  reach  any  degree  of  perfection.  A  topographical 
map  is  a  representation  to  scale  of  the  conformation  of  the 
ground;  it  is  apparent  that  the  actual  conformation  cannot 
be  exactly  reproduced  on  a  map.  A  topographical  map  is 
therefore  a  generalization,  more  or  less,  of  the  terrain.  The 
degree  of  generalization  will  depend :  first,  upon  the  scale  of  the 
map,  the  smaller  the  scale  the  greater  the  generalization;  and 
secondly,  upon  the  degree  of  accuracy  required,  the  larger  the 
number  of  locations  geometrically  determined  the  less  the 
generalisation  and  the  more  accurate  the  map.  It  would  be 
possible  to  determine  every  critical  point  of  the  terrain,  but  a 
topographical  survey  so  made  would  be  so  expensive  that  the 
mapping  of  any  considerable  area  would  be  impracticable. 
Good  sketching,  therefore,  requires  correct  generalization — the 
ability  to  take  both  a  broad  and  a  detailed  view  of  the  terrain  in 
order  to  utilize  the  essential  details  in  the  interpretation  of  the 
important  features  of  the  terrain,  and  to  bring  them  into  proper 
correlation — to  know  in  short  what  details  may  be  omitted  and 
what  must  be  preserved  in  order  to  bring  out  the  predominant 
features. 

The  military  topographer  must  have  a  tactical  knowledge  of 
the  military  uses  and  requirements  of  a  topographical  map  in 
order  to  make  such  a  map.  The  visibility  of  points  and  areas, 
the  character  of  slopes,  the  presence  and  condition  of  all  roads, 
trails,  passes,  streams,  vegetation,  cultivation,  artificial  con- 
structions, etc.,  are  all  essential  to  the  military  uses  of  maps. 


44  Military  Topography  and  Photography 

CONTROL  WORK 

GEODETIC  OPERATIONS.  The  military  topographer  will  seldom 
if  ever,  use  geodetic  methods ;  but  the  skeleton  of  control  for 
accurate  military  surveys  is  usually  the  geodetic  triangula- 
tion  of  the  civil  government,  and  since  the  use  of  this  data 
requires  a  general  knowledge  of  the  subject  matter,  its  opera- 
tions will  be  presented  in  a  general  way  first  so  that  the  military 
topographer  can  make  use  of  the  same. 

GEODESY.  In  a  general  sense  the  first  operation  in  any 
information  survey  is  to  locate  the  territory  to  be  surveyed  at' 
its  true  location  on  the  surface  of  the  earth.  The  determina- 
tion of  the  latitude  and  longitude  of  important  locations  and 
the  areas  of  large  territories  are  the  functions  of  .geodesy. 
Having  determined  the  latitude  and  longitude  of  a  base  point, 
the  geodetic  locations  of  other  important  locations  near  it  are 
computed  from  an  elaborate  system  of  triangulation.  This 
consists  in  accurately  measuring  a  base  line  of  from  four  to 
ten  miles  long,  one  end  of  which  is  the  base  point  whose  coordi- 
nates of  latitude  and  longitude  have  been  determined  by  geodetic 
or  astronomic  methods,  and  upon  this  line  constructing  a  system 
of  triangles.  Any  side  of  any  triangle  so  formed  may  be  used 
as  a  base  to  form  new  triangles,  and  the  system  of  triangles 
can  be  carried  forward  in  any  direction.  This  progression 
should  not,  however,  be  carried  forward  in  any  direction  more 
than  250  miles  from  the  original  base  line.  At  such  a  distance 
a  new  base  line  should  be  measured  and  the  geodetic  coordinates 
of  one  of  its  extremities  determined  by  astronomic  methods,  in 
order  to  check  the  control  work  and  to  carry  the  triangulation 
further.  In  geodetic  and  geological  surveys,  single  triangles 
are  not  allowed;  each  unknown  point  must  be  the  vertex  of  at 
least  three  triangles  with  respect  to  a  known  line.  This  forms 
a  check  on  the  accuracy  of  the  work  and  furnishes  a  means  of 
properly  distributing  any  errors.  In  each  triangle  one  side 
and  the  three  angles  are  known  from  which  the  two  unknown 
sides  can  be  computed.  The  diagram  in  Fig.  16  shows  the 
simplest  form  of  a  geodetic  or  geological  triangulation. 

THE  BASE  LINE.  An  open  and  level  stretch  of  ground  from 
four  to  ten  miles  long  is  selected.  If  this  be  not  available  then 


B 


16 


46  Military  Topography  and  Photography 

a  stretch  of  uniform  slope  is  selected,  or,  if  necessary  broken 
level  stretches  for  the  desired  length  are  selected.  Permanent 
stations  or  monuments  are  erected  at  both  ends,  and  the  latitude 
and  longitude  of  one  of  the  ends  is  determined  by  astronomic 
methods.  The  distance  between  the  two  ends  or  base  stations 
is  then  measured  with  an  invar  or  steel  tape  that  has  been 
calibrated  with  a  standard  for  the  occasion.  Methods  of  pre- 
cision are  followed  to  get  the  measurement  within  the  degree  of 
accuracy  required ;  corrections  are  made  for  tension,  tempera- 
ture, sag,  slope,  and  elevation.  Base  lines  can  be  measured  with 
a  very  high  degree  of  accuracy,  using  either  the  invar  or  steel 
tape.  The  following  shows  the  results  obtained  by  the  Coast 
&  Geodetic  Survey  in  1907  on  six  base  lines  measured  along 
the  98th  Meridian : 

Base  Line  Probable  Error 

Invar  Tape  Steel  Tape 

Point  Isabel 1  in  2,310,000  1   in   1,300,000 

Williamette  .  .  . 3,340,000  1,730,000 

Tacoma .  .  2,980,000  1,630,000 

Stephen     2,940,000  1,120,000 

Brown   Valley    3,110,000  1,420,000 

Royalton     2,460,000  2,260,000 

LATITUDE  DETERMINATION.  Latitude  may  be  determined  in 
a  number  of  ways,  two  are  here  given.  One  of  the  most  precise 
methods  used  in  geodetic  work  is  to  measure  with  a  zenith  tele- 
scope the  zenith  distances  of  two  stars  whose  difference  in 
zenith  distances  is  very  small.  In  practice  a  number  of  sets 
of  such  stars  are  observed  and  their  mean  taken  for  the  latitude 
of  the  station.  Formula:  z=0 — s,  and  z'=s' — 0,  from  which 
0==:!/2(s+s/)+:(/2(z — z')  ;  in  which  0=latitude  of  station,  z  and 
z'=zenith  distances  of  the  two  stars,  and  s  and  s'=their  alti- 
tudes. The  simplest  method  is  to  measure  the  meridian  zenith 
or  altitude  of  a  known  star;  then  0=sdbz.  A  sextant  or  transit 
may  be  used  in  this  method;  the  known  star  may  be  the  Sun, 
Polaris,  or  any  other  recognized  star.  The  declination  and 
right  ascension  of  all  important  stars,  the  sun,  and  the  moon, 
for  the  Greenwich  Meridian  are  given  in  the  Nautical  Almanac. 


ZENITH  TELESCOPE 

Courtesy  of  the  Coast  &  Geodetic  Survey. 


48  Military  Topography  and  Photography 

The  second  method  is  often  used  in  exploratory  surveys  and  in 
reconnaissances,  but  it  is  not  suitable  for  geodetic  work. 

LONGITUDE  DETERMINATION.  The  longitude  of  a  point  is 
found  by  accurately  determining  the  difference  in  time  between 
it  and  another  point  whose  longitude  is  known.  The  other 
point  is  usually  the  Prime  Meridian,  which  requires  that  ac- 
curate Greenwich  time  be  had  at  the  station  whose  longitude  is 
to  be  found.  Such  time  is  kept  on  a  chronometer  or  chrono- 
graph which  has  been  taken  to  and  set  with  one  that  has  ac- 
curate Greenwich  time.  The  time  at  which  a  known  star  tran- 
sits the  meridian  of  the  station  whose  longitude  is  desired,  is 
observed  and  recorded  on  the  chronometer.  The  difference 
between  this  recorded  time  at  which  the  known  star  transited 
the  meridian  of  the  station  and  the  time  at  which  it  transited 
the  Prime  Meridian  (given  in  the  Nautical  Almanac),  is  the 
true  distance  in  time  between  them,  from  which  their  difference 
in  longitude  may  be  directly  reduced  (one  second  in  time  equals 
15  seconds  in  longitude).  In  practice  observations  are  taken 
from  two  stations  some  distance  apart,  one  of  which  has  had  its 
geodetic  coordinates  already  determined,  and  the  time  of  each 
observation  at  each  station  is  recorded  simultaneously  on  the 
chronometer  of  each  station  by  means  of  an  electric  circuit  and 
key  which  gives  a  check  on  the  chronometers  and  the  time; 
several  observations  are  taken  on  the  star  just  before  it  crosses 
the  meridian  and  several  just  after,  rather  than  trying  to 
observe  the  exact  transit,  which  eliminates  personal  equations. 
The  observations  are  taken  on  a  number  of  known  stars,  their 
exact  transits  computed,  and  the  mean  taken  for  the  true  dif- 
ference in  time. 

In  exploratory  surveys  the  transit  of  a  known  star  may  be 
determined  with  sufficient  accuracy  by  a  sextant.  Observations 
both  for  latitude  and  longitude  at  sea  are  made  with  the  sextant 
by  measuring  the  altitude  and  azimuth  of  a  known  star.  Wash- 
ington or  Greenwich  time  is  kept  with  a  chronometer  on  ship 
board. 

TRIANGULATION.  After  the  base  line  has  been  measured  and 
the  coordinates  of  one  of  its  stations  (ends)  determined,  the 


50  Military  Topography  and  Photography 

azimuth  of  the  Base  Line  is  determined  by  sun  observations. 
Triangulation  stations  are  then  selected  to  carry  the  triangula- 
tion  forward.  These  stations  are  as  a  rule  from  eight  to  ten 
miles  apart  and  the  most  prominent  points  of  the  terrain  are 
selected  for  them  (sometimes  their  distance  apart  is  much 
greater)..  No  point  is  selected  as  a  station  which  would  form 
with  the  base  line  from  which  its  location  is  to  be  determined 
a  triangle  in  which  any  angle  is  less  than  30°  nor  more  than 
120°,  the  more  equiangular  the  better.  Each  station  is  marked 
with  a  permanent  monument  which  is  usually  a  cement  slab  with 
a  triangulation  mark  on  top  of  it.  The  monument  is  placed 
sufficiently  deep  to  eliminate  disturbances  by  frost  and  local 
causes.  A  tower  or  tripod  marker  is  then  erected  over  the 
monument,  of  such  height  or  size  that  a  theodolite  may  be  set 
up  either  under  it  or  on  top  of  it ;  the  marker  for  sighting  on 
from  adjacent  triangulation  stations  consists  generally  of  a 
prism  frame  covered  with  white  cloth,  although  a. mirror  or 
heliograph  marker/ is  sometimes  used;  in  very  level  countries 
towers  of  considerable  height  are  necessary.  Each  triangulation 
station  is  then  occupied  with  a  theodolite,  and  the  angles  of  each 
triangle  are  measured  by  the  method  of  repetition  or,  in  less 
accurate  work,  by  direction.  The  sum  of  the  angles  of  any 
triangle  should  not  vary  from  180°  by  more  than  15".  The 
spherical  excess  is  deducted  and  the  angles  adjusted  by  the 
method  of  least  squares:  for  geodetic  purposes  triangles  must 
be  considered  and  adjusted  as  spherical  triangles.  From  the 
known  side  and  three  known  angles  of  each  triangle,  the  other 
two  sides  are  computed,  and  from  this  the  geodetic  coordinates 
of  each  triangulation  station  are  determined. 

The  geodetic  data  of  triangulation  stations  include : 

(1)  The  Elevation  above  mean  sea  level  of  each  station. 

(2)  The  Latitude  of  each  station. 

(3)  The  Longitude  of  each  station. 

(4)  The  Azimuth  of  each  two  adjacent  stations. 

(5)  The  Back-Azimuth  of  each,  two  adjacent  stations. 

(6)  The  Distance  between  each  two  adjacent  stations. 


THEODOLITE 

Courtesy    of    the    Coast    &    Geodetic    Survey. 


52  Military  Topography  and  Photography 

Example : 


Station 

Latitude 
Longitude 

Sees,   in 
Meters 

Azimuth 

Back- 
Azimuth 

To 

Station 

Distance 

Two     Tips 

39°43 

'01" 

37.8 

19°03 

'20" 

198°51' 

38" 

Mt.    Como 

81,622 

M 

119°09 

'31" 

1330.0 

198°13 

'31" 

288°01' 

42" 

Pah-Rah 

27,774 

M 

280«>13 

'19" 

100°48' 

57" 

Carson  Sk 

81,253 

M 

Mucca 

39°58 

'35" 

1983.7 

288°10 

'23" 

109»08' 

18" 

Carson   Sk 

136,436 

M 

119°44 

'36" 

854.9 

331°55 

'11" 

153°31' 

22" 

Mt.    Grant 

176,388 

M 

347°33 

'02" 

167°43' 

20" 

Mt.  Como 

108,514 

M 

Seconds  of  latitude  and  longitude  are  also  given  in  their 
equivalent  in  feet  or  meters,  and  the  magnitude  of  each  second 
of  latitude  and  longitude  varies  with  the  latitude  of  the  station. 
For  values  see  Table  V. 

CONTROL  WORK  OF  MILITARY  SURVEYS 

A  military  survey  may  be  made  of  a  territory  in  which  the 
Coast  and  Geodetic  Survey  or  the  Geological  Survey  has 
already  established  triangulation  stations,  or  it  may  be  in  a 
territory  that  has  never  been  visited  by  either  of  these  surveys. 
In  the  former  case,  the  military  topographer  will  be  relieved  of 
much  preliminary  control  work,  by  utilizing  their  data,  which 
is  always  available  to  him.  In  foreign  territories,  such  data 
of  their  government  will  perhaps  not  be  available,  but  topo- 
graphic work  in  such  places  will  usually  be  confined  to  rapid 
military  sketches  aided  by  such  maps  as  can  be  obtained. 

CONTROL  WORK  WITH  GEODETIC  TRIANGULATION  STATIONS 
The  geodetic  stations  are  usually  so  far  apart  that  they  must 
be  supplemented  with  other  triangulation  stations  in  order  that 
sufficient  stations  will  be  visible  to  all  probable  plane  table 
resection  points.  The  number  of  the  probable  plane  table  re- 
section points  and  their  distribution  will  depend,  first,  upon 
the  closeness  of  the  terrain,  and  second,  upon  the  degree  of 
accuracy  required.  There  should  be  at  least  three  triangula- 
tion flags  visible  to  each  resection  station.  The  visibility  of 
triangulation  stations  will  depend  upon  the  closeness  of  the 
terrain  and  the  distance  at  which  they  are  used.  Triangulation 
stations  will  usually  be  marked  with  flags  and  with  the  average 
conditions  of  daylight  the  limit  of  visibility  will  be  about  four 
miles.  When  the  terrain  is  close  triangulation  flags  which  are 
near  a  resection  station  are  quite  often  invisible  on  account  of 
an  intervening  hill  or  woods,  but  in  such  a  case  resection  can 


S  Q 


54  Military  Topography  and  Photography 

often  be  made  on  more  distant  triangulation  flags.  Triangula- 
tion  flags  established  about  every  two  miles  will  usually  suffice 
for  the  average  terrain.  If  entirely  open,  fewer  flags  will  suf- 
fice ;  or  if  there  are  a  few  commanding  hills ;  if  the  ground  be 
level  or  covered  with  trees,  it  may  be  necessary  to  supplement 
the  geodetic  stations  with  control  traverses  for  the  requisite 
framework.  No  normal  system  of  triangulation  stations  can 
be  made,  but  the  diagram  in  Fig.  17  will  give  the  general  idea 
of  triangulation  control. 

PRELIMINARY  RECONNAISSANCE.  The  topographer  should 
first  make  a  preliminary  reconnaissance,  mounted  if  possible,  of 
his  territory,  from  which  to  get  a  general  idea  of  the  conforma- 
tion of  the  ground,  and  of  the  character  of  the  vegetation  in 
order  to  select  the  hills  and  ridges  on  which  he  should  place  his 
triangulation  stations,  and  to  intelligently  plan  his  work. 

Stations  whose  locations  are  determined  by  triangulation 
methods  are  classified  into  three  divisions  according  to  the 
degree  of  accuracy  by  which  they  are  determined,  (1)  Primary 
Triangulation  Stations,  (2)  Secondary  Triangulation  Stations, 
and  (3)  Tertiary  Triangulation  Stations.  This  classification 
must  not  be  confused  with  the  Primary,  Secondary,  and  Ter- 
tiary Triangulation  of  the  Coast  and  Geodetic  Survey,  which  is 
entirely  different. 

Primary  Stations  consist  of  those  triangulation  stations 
which  are  determined  with  the  greatest  accuracy,  and  upon 
which  the  work  is  carried  forward  and  from  which  other  pri- 
mary stations  are  determined.  Secondary  Stations  are  those 
triangulation  stations  which  are  not  so  accurately  determined 
as  primary  stations — no  corrections  are  made  for  adjustment, 
but  from  them  tertiary  stations  are  allowed  to  be  determined 
by  resection.  Tertiary  Stations  are  commonly  called  plane 
table  stations  and  are  determined  by  resection  from  primary 
and  secondary  stations.  Their  difference  will  be  more  easily 
seen  after  their  methods  of  determination  are  explained ;  the 
determinations  of  tertiary  stations  are  explained  under  "Plane 
Table  Methods." 

PRIMARY  TRIANGULATION.  In  the  preliminary  reconnais- 
sance the  topographer  from  the  geodetic  data,  should  have  been 


GEODETIC  STATIOKS 
OTHER  PRIMARY  STATIONS 
SECONDARY  STATIONS' 


FIG.   17 


56  Military  Topography  and  Photography 

able  to  find  or  recover  the  geodetic  triangulation  stations  with- 
in his  territory,  and  he  should  then  have  each  of  these  stations 
marked  with  a  flag.  This  flag  should  be  about  a  yard  wide  by 
about  a  yard  and  a  quarter  long — muslin  is  best ;  the  flag  should 
be  principally  white,  but  it  will  be  more  easily  picked  up  at  a 
distance  if  the  last  quarter  of  a  yard  of  the  flag  is  red ;  the  flag 
staff  should  be  a  straight  pole  from  20  to  30  feet  long — bamboo 
makes  an  excellent  flag  staff,  and  it  should  be  braced  at  its  base 
with  a  tripod. 

He  should  then  select  a  hill  or  knoll  with  respect  to  the  two 
most  suitable  adjacent  geodetic  stations,  which  is  visible  to  both 
and  is  so  located  with  respect  to  them  that  an  obtuse  triangle 
will  be  formed  in  which  no  angle  is  less  than  30°  nor  more  than 
120°.  The  distance  between  the  two  adjacent  geodetic  stations 
will  always  be  the  long  side  of  this  triangle,  and  the  knoll  should 
be  so  selected  that  the  two  other  sides  will  be  as  short  as  the 
conditions  with  respect*  to  the  angles  will  permit.  This  will  give 
two  sides  from  five  to  six  miles  long  each  and  upon  which  a  sys- 
tem of  triangles  can  be  extended  whose  sides  will  be  from  four 
to  six  miles  long,  on  the  average.  In  most  military  survey  work 
simple  triangles  will  suffice,  but  the  system  should  not  be  ex- 
tended very  far;  such  a  system  is  not  suitable  for  adjustment 
to  other  geodetic  stations  and  the  azimuth  that  is  carried  for- 
ward in  longitude  will  constantly  increase  in  error. 

Prominent  points  should  have  been  selected  in  the  preliminary 
reconnaissance  for  the  primary  triangulation  stations;  these 
stations  should  be  from  four  to  five  miles  apart;  the  angles  of 
each  triangle  are  measured  by  the  method  of  repetition  and 
their  sum  should  not  vary  from  180°  by  more  than  15";  this 
error  is  distributed  equally  with  the  three  angles. 

CONTROL  WORK  WITHOUT  GEODETIC  DATA.  When  the  topog- 
rapher does  not  know  and  has  no  data  of  the  geodetic  coordi- 
nates of  any  point  within  his  territory,  there  are  a  number  of 
ways  that  he  may  proceed  in  his  work.  It  should  be  remem- 
bered that  in  any  survey,  there  must  be  a  point  of  beginning;  if 
there  is  none  that  has  been  established,  the  topographer  should 
arbitrarily  select  a  Base  Point  and  do  his  survey  work  with 
respect  to  it;  the  survey  work  can  be  just  as  accurate  and  later 


Military  Topography  and  Photography  57 

it  may  be  able  to  determine  the  geodetic  coordinates  of  that 
point,  and  then  the  survey  work  can  be  transferred  to  a  pro- 
jection sheet. 

PRELIMINARY  RECONNAISSANCE.  A  preliminary  reconnais- 
sance should  be  made  as  in  the  preceding  case — to  become 
acquainted  with  the  ground  and  to  select  triangulation  stations. 

BASE  LINE.  When  there  are  no  geodetic  stations,  it  will  be 
necessary  to  measure  a  Base  Line  upon  which  to  form  and 
extend  the  primary  triangulation.  This  base  line  should  be 
from  two  to  five  miles  long,  according  to  the  extent  and  charac- 
ter of  the  survey.  The  accuracy  generally  required  in  the 
measurement  of  a  Base  Line  in  this  work  is  1 :100,000 ;  but  if 
only  a  small  area  is  to  be  surveyed  and  no  other  work  is  going 
to  be  based  upon  it,  1 :10,000  will  suffice.  The  instructions 
given  by  the  authority  ordering  the  survey  will  cover  this.  For 
measurement  of  Base  Line,  see  page  238. 

One  end  of  the  base  line  should  be  the  selected  base  point; 
and  its  elevation  should  be  determined  or  assumed.  If  the  terri- 
tory be  near  the  seashore,  or  near  a  bench  mark  a  series  of 
levels  (spirit)  may  be  run  from  it.  To  determine  the  mean 
sea  level,  the  level  should  be  set  up  at  a  convenient  point  along 
the  beach  and  the  elevation  of  the  surface  of  the  sea  with  respect 
to  the  level  should  be  determined  every  half  hour  throughout  the 
day.  The  mean  of  the  readings  will  give  the  height  of  the  level 
above  mean  sea  level. 

The  primary  triangulation  is  now  carried  on  as  explained  in 
the  preceding  case. 

SECONDARY  TRIANGULATION.  At  the  same  time  that  the 
points  are  selected  for  the  primary  stations,  other  points  are 
selected  for  the  necessary  secondary  stations  and  the  same 
marked  with  a  flag  unless  a  church-spire,  house-chimney,  or 
other  distinctive  object  has  been  selected  for  a  secondary  sta- 
tion. Secondary  stations  are  located  by  taking  intersecting 
rays  on  them  from  three  primary  stations.  These  intersecting 
rays  are  true  azimuth  rays,  the  azimuth  being  obtained  from  the 
Geodetic  data  or  determined  by  sun  azimuth.  The  location  of 
secondary  stations  can  be  determined  by  plotting  those  inter' 
secting  azimuth  rays  on  the  field  sheet.  The  coordinates  of 


58  Military  Topography  and  Photography 

primary  stations  are  computed  and  the  stations  are  located  on 
the  field  sheet  by  plotting  their  coordinates.  Secondary  sta- 
tions are  not  occupied  with  the  transit. 

TERTIARY  TRIANGULATION.  Tertiary  Stations  are  those 
whose  map  positions  are  determined  by  resection.  Their  deter- 
mination is  fully  treated  under  Plane  Table  Operations. 

CONTROL  TRAVERSES.  In  level  sections,  in  wooded  areas,  and 
in  deep,  narrow  valleys,  triangulation  flags  cannot  be  seen,  and 
in  such  places  it  is  necessary  to  run  control  traverses  for  the 
framework  of  control;  the  area 'included  within  each  traverse 
will  vary;  for  secondary  control  within  these  areas,  needle  and 
back-sight  traverses  are  run.  Control  traverses  are  often  used 
in  civil  surveys  for  the  location  of  roads  and  streams,  but  such 
use  of  them  will  be  rather  the  exception  in  military  surveys ; 
for  such  work  needle  and  back-sight  traverses  checked  by 
triangulation  will  usually  suffice. 

Controls  needle,  and  back-sight  traverses,  may  replace  all 
triangulation  except  the  Geodetic,  and  even  that  in  territories 
where  it  does  not  exist. 

TRIANGULATION  LEVELING.  The  difference  in  elevation  be- 
tween two  points  is  equal  to  their  horizontal  distance  apart 
times  the  tangent  of  their  slope  angle.  In  a  triangulation  sys- 
tem, the  slope  or  vertical  angles,  from  at  least  two  known  sta- 
tions to  each  unknown  station,  are  measured  and  the  slope  angle 
between  every  two  stations  is  measured  in  both  directions. 
This  gives  a  very  accurate  check  upon  vertical  or  slope  angles 
from  which  differences  in  elevations  can  be  computed.  Allow- 
ance should  be  made  for  the  curvature  of  the  earth  (7.92"XD2) . 

PLANE  TABLE  OPERATIONS 

The  Plane  Table  consists  of  two  general  parts :  ( 1 )  a  plot- 
ting board,  about  24"X30",  supported  on  a  tripod,  and  (2)  an 
alidade  with  telescope  attached.  The  plotting  board  has  at- 
tached to  it  on  its  under  side  supporting  apparatus  by  means 
of  which  it  may  be  leveled  and  revolved  in  a  horizontal  plane  the 
same  as  a  transit.  There  are  either  three  or  four  leveling  screws 
and  a  clamp  screw  which  controls  the  vertical  axis  of  the  plane 
table;  the  clamp  screw  has  a  tangent  screw  attached  to  it  for 


Military  Topography  and  Photography 


59 


the  more  delicate  orientation  of  the  plane  table.  The  telescope 
can  be  plunged  in  a  vertical  plane,  and  containing  stadia  wires, 
can  be  used  in  telemetric  work.  The  edge  of  the  alidade  ruler 
and  the  line  of  collimation  of  the  telescope  are  parallel.  A 
magnetic  box  or  trough  compass  and  leveling  tube  are  also 
attached  to  the  plotting  board. 

The  plane  table  is  the  ideal  instrument  for  topographical 
plotting,  but  it  is  very  clumsy  and  heavy  to  carry  about  in  the 
fiejd.  All  the  operations  that  can  be  made  with  a  transit  can 


*  PLANE  TABLE  AND  ALIDADE 

be  made  with  the  plane  table,  but  the  observations  are  plotted 
directly  instead  of  being  first  recorded.  With  the  plane  table, 
both  the  observations  and  plotting  can  be  performed  by  one 
man.  Military  surveys,  however,  will  usually  be  made  with 
transit  and  sketching  board,  with  one  man  acting  as  transit 
man  and  another  as  topographer. 

TOPOGRAPHIC  METHODS.  There  are  a  number  of  different 
methods  by  which  topographic  surveys  may  be  made.  These 
methods  may  all  be  divided  into  two  general  classes:  (1)  those 
in  which  the  area  is  subdivided  into  squares  by  means  of  inter- 
secting lines,  the  elevations  of  the  intersections  being  deter- 

*Courtesy  of  W.  &  L.  E.  Gurley. 


60  Military  Topography  ^and  Photography 

mined,  and  (2)  those  in  which  the  locations  and  elevations  of 
certain  critical  or  controlling  points  are  determined  geomet- 
rically. In  the  former  method,  called  checkerboard  system,  the 
elevations  of  the  intersections  are  written  near  the  correspond- 
ing intersections  on  the  plotting  sheet,  and  contours  are  drawn 
between  proper  elevations.  The  contouring  in  this  method  is 
usually  done  in  the  office  and  the  resulting  topographic  map 
represents  the  terrain  only  in  a  very  general  sense,  unless  the 
intersections  are  taken  quite  close  together.  Engineering  sur- 
veys for  cuts  and  fills  are  so  made,  but  such  surveys  are  too 
expensive  for  information  surveys. 

In  the  latter  method  the  measurement  may  be  made  either 
(1)  with  chain  and  level,  (2)  with  transit  .and  stadia,  (3)  with 
plane  table  and  stadia,  (4)  with  transit,  sketching  board  and 
stadia,  and  (5)  with  photo-theodolite.  Unless  otherwise  special- 
ly stated  all  plane  table  operations  will  be  explained  for  the 
plane  table  and  stadia  method  in  this  book,  these  explanations 
with  slight  modification  can  be  applied  to  the  transit  and  sketch- 
ing board  method.  Photo-topographic  methods  will  be  taken 
up  separately. 

PREPARATION  OF  FIELD  SHEETS.  The  paper  on  which  the 
field  plotting  is  done  should  be  so  prepared  as  to  reduce  distor- 
tion from  expansion  and  contraction.  Ordinary  paper  expands 
and  contracts  along  the  grain  greater  than  across  'the  grain 
of  the  paper,  which  distorts  the  paper  out  of  proportion  and 
introduces  errors  that  cannot  be  determined  and  allowed  for. 
The  U.  S.  Geological  Survey  uses  two  sheets  of  paragon  paper 
mounted  with  the  grain  at  right  angles  and  with  cloth  between 
them.  The  Coast  &  Geodetic  Survey  uses  Whatman's  cold 
pressed  hand  made  antiquarian  paper  backed  with  muslin. 
Tracing  paper  and  linen  may  be  used  for  less  accurate  work. 

The  Scale  will  be  prescribed  by  the  authority  ordering  the 
survey.  Nothing  is  gained  by  making  the  scale  too  small  even 
though  the  area  to  be  surveyed  is  large.  Military  maps  require 
many  details  not  required  in  civil  maps,  and  if  the  scale  be  too 
small,  the  minute  plotting  involved  proves  a  tax  on  the  topog- 
rapher and  is  a  great  time  consumer.  It  should  be  remembered 
that  a  map  cannot  be  enlarged  without  multiplying  and  increas- 
ing the  size  of  all  errors,  while  in  reducing  a  map  the  errors  are 


Military  Topography  and  Photography 


61 


reduced  also.  These  considerations  will  require  that  the  scale 
for  field  plotting  should  not  be  smaller  than  1-25,000.  Upon 
the  other  hand  if  the  scale  be  too  large  then  a  sufficient  number 
of  triangulation  stations  cannot  be  plotted  on  the  field  sheet 
at  the  same  time  so  as  to  do  efficient  field  work.  This  will 
generally  limit  the  scale  to  one  not  larger  than  1-10,000.  For 
general  topographic  work,  a  scale  of  1-21,120,  or  three  inches 
to  one  mile  will  be  the  best  for  field  plotting. 


FIG.  18 — SKETCHING  BOARD 


62  Military  Topography  and  Photography 

PLOTTING  THE  COORDINATE  LINES.  Field  sheets  for  large 
areas  should  be  plotted  on  poly  conic  projections,  but  for  field 
sheets  wfyich  represent  an  area  several  miles  square,  the  coordi- 
nates of  longitude  and  latitude  can  both  be  plotted  as  parallel 
lines.  If  so  plotted  each  minute  of  longitude  and  latitude  are 
to  be  represented  by  meridian  and  parallel  lines,  respectively,  on 
the  field  sheet ;  a  line  YY'  is  drawn  perpendicularly  across  the 
center  of  the  field  sheet ;  a  line  XX'  is  then  drawn  perpendicular 
to  line  YY'  near  the  lower  edge  of  the  field  sheet ;  from  Table 
V  the  value  of  one  minute  in  latitude  is  found,  and  that  dis- 
tance is  measured  off  along  line  YY',  as  many  times  as  it 
is  contained  in  the  line  YY' ;  through  these  points,  lines 
XiX'i,  X2X'2,  etc.,  are  drawn  parallel  to  XX';  the  value  in 
feet  of  one  minute  of  longitude  is  then  found  from  the  same 
table,  and  that  distance  is  measured  off  along  line  XX'; 
through  these  points  lines  YiY'j,  Y2Y'2,  etc.,  are  drawn  parallel 
to  YY'.  The  longitude  and  latitude  of  coordinate  lines  should 
be  marked  on  the  field  sheet  to  avoid  errors  iir  plotting  triangu- 
lation  stations. 

PLOTTING  TRIANGULATION  STATIONS.  The  coordinates  of 
triangulation  stations  are  computed  and  recorded  with  the 
seconds  of  longitude  and  latitude  expressed  in  meters.  From 
the  nearest  minute-parallel  of  latitude,  the  distance  of  seconds 
in  meters  is  laid  off  on  the  proper  YnYn  line;  similarly  from 
the  nearest  minute-meridian  of  longitude,  the  distance  of  sec- 
onds of  longitude  in  meters  is  measured  from  the  proper  XnXn 
line.  Through  these  points  lines  parallel  to  the  YnYn  and 
XnXn  lines  are  drawn.  Their  intersection  is  the  location  of  the 
triangulation  station.  In  the  diagram,  Fig.  18,,  a  triangula- 
tion station  whose  coordinates  are  40°  15'  20M.  North  and 
80°  32'  7.9M.  West  has  been  plotted. 

Secondary  triangulation  stations  are  plotted  from  intersect- 
ing azimuth  rays  from  primary  triangulation  stations.  The 
plane  table  is  set  up  over  a  primary  station,  the  plotting  board 
correctly  oriented,  the  alidade  sighted  on  the  secondary  flags 
and  pencil  rays  drawn  along  the  edge  of  the  alidade  ruler  after 
each  sight.  Such  sights  should  be  taken  from  at  least  three 
primary  stations ;  the  intersection  of  the  three  rays  in  the  same 


Military  Topography  and  Photography  63 

point  gives  a  check  on  the  accuracy  of  the  orientation  of  the 
plane  table  and  the  work. 

SETTING  UP  THE  PLANE  TABLE.  In  setting  up  a  plane  table 
there  are  three  things  to  be  accomplished:  (l).to  get  the  plane 
table  level,  (2)  to  get  a  certain  point  on  the  plotting  board  right 
over  its  corresponding  position  on  the  ground,  and  (3)  to  orient 
the  board. 

(1)  Setting  Up  the  Plane  Table:     The  plane  table  should 
be  placed  so  that  the  map  station  and  its  ground  station  are  as 
nearly  as  practicable  in   the   same  vertical  line.      To  do  this 
clasp  the  two  near  legs  of  the  tripod  in  the  hands  and  with  an 
outward  sweep  swing  the  loose*leg  to  its  proper  place  so  that 
when  the  tripod  legs  form  an  angle  of  about  30°   with  each 
other,  the  map  station  is  over  the  ground  station;    the  plane 
table  should  be  leveled  as  much  as  possible  by  moving  the  proper 
tripod  leg  in  or  out.     If  the  topographer  has  misjudged  the 
proper  position  for  the  tripod  legs,  the  plane  table  must  be 
moved  bodily  to  its  proper  place.     With  practice  the  topog- 
rapher will  be  able  to  judge  very  close  to  the  proper  position 
of  rest  for  the  loose  leg  of  the  tripod. 

(2)  Leveling  the  Plane  Table:     The  plane  table  is  leveled 
the  same  as  a  transit.     The  vertical  plane  of  each  pair  of  level- 
ing screws  must  be  marked  on  the  board  so  that  the  same  posi- 
tion may  be  brought  over  the  same  pair  of  leveling  screws  each 
time.     The  opposite  screws  of  each  pair  must  be  turned  simul- 
taneously and  either  away  from  or  towards  each  other,  the  left 
thumb  moving  in  the  direction  in  which  it  is  desired  the  bubble 
to  go. 

(3)  Orienting  the  Plane  Table:     Loosen  the  clamp  screw 
and  orient  the  plane  table  with  the  eye ;   then  tighten  the  clamp 
screw  and  orient  the  table  for  fine  adjustment  by  means  of  the 
tangent  screw.     If  the  table  is  oriented  by  means  of  the  com- 
pass,  the   north  and  south  line   on   the   field   sheet   should  be 
brought  into  coincidence  with  the  true  north  and  south  line  of 
the  compass.      For   a  more  detailed  description  of  the  plane 
table,  its  adjustments,  etc.,  see  Engineer  Field  Manual,  Part  I. 
For  orientation  by  resection,  see  pp.  67-73. 


64  Military  Topography  and  Photography 

INSTRUMENT  STATIONS.  Any  station  where  the  plane  table 
is  set  up  is  an  instrument  station.  If  possible  the  instrument 
station  should  be  in  such  a  commanding  place  that  the  sur- 
rounding terrain  within  range  of  stadia  readings,  about  one- 
half  mile,  may  be  seen.  An  instrument  station  should  also  be  a 
critical  point  whenever  possible.  The  location  of  plane  table 
stations  may  be  determined  ty  (1)  resection,  or  (2)  meanda- 
tion. 

(1)  Location  by  Resection:  Resection  is  the  locating  of 
an  unknown  occupied  point  by  means  of  intersecting  lines  of 
azimuths  from  known  visible  plotted  points  of  the  terrain. 
There  are  a  number  of  methods  that  may  be  employed,  and  a 
graphic  explanation  of  them  will  now  be  made. 

The  Two-Point  Problem:  Having  given  two  visible  plotted 
points,  a  and  b,  to  determine  the  map  position  of  an  unknown 
occupied  station  X.  (1)  By  compass  orientation. 

Set  up,  level,  and  orient  the  plane  table  at  the  unknown  sta- 
tion X ;  pivot  the  alidade  about  the  plotted  point  a,  sighting  its 
station  A,  and  draw  a  light  pencil  ray  along  the  edge  of  the 
alidade  ruler;  similarly  pivot  the  alidade  about  the  plotted 
point  b,  sighting  its  station  B,  and  draw  a  light  pencil  ray. 
Where  these  two  rays  intersect  is  the  map  position  x  of  the 
unknown  station  X.  (Fig.  11  a.) 

The  Two-Point  Problem — Continued:  (2)  By  graphic 
orientation.  (Fig.  19.) 

This  method  is  used  when  only  two  plotted  points  can  be  seen 
and  the  "two-point  method  by  compass  orientation"  is  not 
sufficiently  accurate.  Set  the  plane  table  up  at  an  auxiliary 
point  Y,  selected  so  that  points  A,  B,  X,  and  Y  form  a  quad- 
rilateral, and  orient  by  compass  or  estimation;  fasten  a  piece 
of  blank  tracing  paper  on  the  plane  table  and  arbitrarily  select 
on  it  a  point  yt  to  represent  the  map  position  of  station  Y; 
pivot  the  alidade  ruler  on  point  yi,  sight  on  points  A,  B,  and  X, 
and  draw  pencil  rays  y1a1,y1bi,  and  yiXi ;  now  set  up  at  sta- 
tion X,  orient  by  placing  alidade  ruler  along  the  ray  yi  x1?  and 
sighting  back  on  Y;  select  a  point  xt  on  ray  yiXi  to  represent 
the  map  position  of  the  unknown  station  X;  pivot  the  alidade 
ruler  on  point. xl5  sight  on  points  A,  B,  and  Y,  and  draw  pencil 


r  on  SKetch 
V  onb-   b'3  on  ba. 


FIG.  19 


66  Military  Topography  and  Photography 


rays  Xiji,  x^x,  and  xjbi  :  draw  a  line  aibi  connecting  the  inter- 
secting points  ax  and  bt,  and  a  quadrilateral  ajbijiXi  will  be 
formed  on  the  tracing  paper  which  is  exactly  similar  to  the 
quadrilateral  ABYX  on  the  ground;  unfasten  the  tracing 
paper  and  place  it  on  the  field  sheet  so  that  point  bi  coincides 
with  the  plotted  point  b,  and  the  line  ajbi  passes  through  the 
plotted  points  a  and  b  ;  now  place  the  alidade  ruler  along  the 
line  Xxbi  and  turn  plane  table  so  that  point  B  is  sighted  and 
the  board  will  be  correctly  oriented  ;  remove  the  tracing  paper, 
pivot  on  point  a  and  by  sighting  A  draw  a  pencil  ray  ax,  simi- 
larly pivot  on  b  and  by  sighting  B  draw  a  pencil  ray  bx;  the 
intersection,  x,  of  these  two  rays  is  the  map  position  x  of  the 
unknown  station  X. 

The  quadrilateral  aJbxXiyj  obtained  by  this  method  is  not  the 
same  size  as  the  quadrilateral  abxy  which  would  be  obtained 
by  connecting  the  actual  map  positions  of  points  A,  B,  X,  and 
Y  ;  but  it  is  similar  in  shape  to  it  as  well  as  the  natural  quadri- 
lateral ABXY,  and  therefore,  angle  ajbiXx  equals  angle  abx, 
and  also  ABX.  It  should  be  remembered  that  points  on  the 
ground  are  represented  by  capital  letters,  while  their  map 
positions  are  represented  by  small  letters. 

THE  THREE-POINT  PROBLEM.  Unless  the  plane  table  can 
be  oriented  accurately  by  compass,  two  known  plotted  points 
will  not  be  sufficient  except  when  the  board  is  oriented  by 
graphic  orientation.  This  for  the  reason  that  any  three  points 
(the  two  known  plotted  stations  and  the  unknown  station)  not 
in  the  same  straight  line  determine  a  circle,  and  the  unknown 
station  whose  map  position  is  determined  by  intersecting  rays 
from  the  two  known  plotted  points  may  have  any  location  on 
that  circle. 

The  plane  table  can  be  oriented  by  the  compass  to  within 
about  five  minutes  of  the  magnetic  azimuth,  and  since  this  error 
may  be  either  plus  or  minus,  a  maximum  error  of  ten  minutes 
is  to  be  expected  in  the  orientation  of  the  plane  table  by  the 
compass.  For  this  reason,  orientation  by  the  compass  is  not 
permitted  in  the  more  accurate  work;  either  the  two-point 
problem  by  graphic  orientation,  or  the  three-point  problem  is 
then  used.  The  latter  is  the  better,  for  the  intersection  of  three 


Military  Topography  and  Photography  67 

rays  in  the  same  point  forms  a  check  on  the  accuracy  of  the 
work.  The  Solar  Attachment  can  also  be  used  for  plane  table 
orientation. 

The  Three-Point  Problem:  Having  given  three  visible 
plotted  points,  a,  b,  and  c,  to  determine  the  map  position  of  an 
unknown  station  X.  (1)  By  Compass  Orientation.  (Fig, 
lib.) 

Set  up,  level,  and  orient  the  plane  table  (by  compass  or  esti- 
mation) at  the  unknown  station  X;  pivot  the  alidade  ruler 
\bout  the  plotted  points,  a,  b,  and  c,  sighting  their  respective 
stations  A,  B,  and  C,  and  draw  rays  as  explained  in  the  two- 
point  problem.  If  the  three  ra*ys  thus  drawn  intersect  in  the 
same  point,  the  table  is  correctly  oriented;  if  not,  revolve  the 
board  slightly  to  the  right  or  left  by  means  of  the  tangent 
screw,  draw  new  rays  and  repeat  the  operation  until  the  three 
rays  do  intercept  in  the  same  point.  The  orientation  of  the 
board  and  the  determination  of  the  map  position  by  this  method 
can  be  greatly  quickened  by  means  of  the  Coast  Survey  Solu- 
tions. 

COAST  SURVEY  SOLUTIONS  OF  THREJE-POINT  PROBLEMS 

In  the  Coast  Survey  solutions,  the  triangle  formed  by  the 
three  visible  plotted  points  is  known  as  the  Great  Triangle; 
the  circle  determined  by  these  three  points  as  the  Great  Circle; 
and  the  small  triangle  formed  by  the  intersecting  rays  when 
they  do  not  meet  in  the  same  point  as  the  Triangle  of  Error. 
(Fig.  20.) 

When  the  unknown  station  is  on  or  near  the  Great  Circle, 
its  position  is  indeterminate. 

Condition  1:  When  the  unknown  station  X  falls  within  the 
Great  Triangle,  the  map  point  x  is  within  the  Triangle  of 
Error.  Solution :  //  the  ray  from  any  of  the  visible  plotted 
stations  falls  to  the  right  of  the  intersection  of  the  rays  from 
the  other  two  visible  plotted  stations,  turn  the  plane  table  to 
the  left,  and  vice  versa.  Fig.  21. 

When  the  unknown  station  X  falls  without  the  Great  Tri- 
angle the  map  point  x  is  without  the  Triangle  of  Error,  and  to 
the  right  or  left  of  it,  according  as  the  plane  table  is  out  of 
position  to  the  left  or  right.  Solution:  //  the  ray  from  the 


FIG.  20 
0  =GREAT  CIRCLE. 

a  b  C^GREAT  TRIANGLE. 
x  y  z— TRIANGLE  OF  ERROR. 


FIG.  21 
DOTTED  BOARD  CORRECTLY  ORIENTED 


70 


Military  Topography  and  Photography 


middle  visible  station  falls  to  the  left  of  the  intersection  of  the 
rays  from  the  other  two  visible  stations,  revolve  the  plane  table 
to  the  right,  and  vice  versa.  Fig.  22. 


B 


FIG.  22 


Condition  2:  When  the  unknown  station  X  falls  within  any 
of  the  three  segments  between  the  Great  Triangle  and  the  Great 
Circle,  the  map  point  x  is  on  the  side  of  the  ray  from  the  middle 
station  opposite  to  that  of  the  intersection  of  the  rays  from  the 


Military  Topography  and  Photography 


71 


other  two  stations.  Solution :  If  the  ray  from  the  middle 
station  is  to  the  right  of  tJie  intersection  of  the  rays  from  the 
other  two  stations,  turn  the  plane  table  to  the  right,  and  vice 
versa.  Fig.  23. 


FIG.  23 

Condition  3:  When  the  unknown  station  X  is  without  the 
Great  Circle  and  within  the  sector  of  an  angle  (produced)  of 
the  Great  Triangle,  the  map  position  x  is  on  the  same  side  of 
the  ray  from  the  middle  station  as  the  intersection  of  the  rays 
from  the  other  two  stations  is.  Solution:  //  the  ray  from  the 
middle  station  is  to  the  right  of  the  intersection  of  the  rays  from 
the  other  two  stations,  turn  the  plane  to  the  left,  and  vice  versa. 
Fig.  22. 


72 


Military  Topography  and  Photography 


Condition  4:  When  the  unknown  point  X  is  without  the 
Great  Circle  and  the  middle  point  is  on  the  near  side  of  a  line 
joining  the  other  two  stations,  the  map  position  x  is  without  the 
Triangle  of  Error,  and  the  ray  from  the  middle  station  lies 


FIG.  24. 


between  it  and  the  intersection  of  the  rays  from  the  other  two 
stations.  Solution :  When  the  ray  from  the  right  hand  station 
is  uppermost,  turn  the  plane  table  to  the  right  and  vice  versa. 
Fig.  24. 


Military  Topography  and  Photography  73 

For  a  more  complete  discussion  of  the  above  solutions  see 
A  Plane  Table  Manual  by  D.  B.  Wainwright,  in  Coast  and 
Geodetic  Survey  report  for  1905. 

The  Three-Point  Problem,  Continued:  (2)  By  Graphic 
Orientation — BesseVs  Method.  Fig.  25. 

(1)  Set  up,  level  and  orient  approximately  by  estimation 
or  compass;  (2)  unclamp,  and  with  the  alidade  ruler  passing 
through  c  and  a,  turn  the  plane  table  to  such  a  position  that 
point  A  may  be  seen  through  the  telescope;  (3)  clamp,  and 
pivot  the  alidade  about  the  point  c,  sighting  point  B  and  draw 
an  indefinite  ray ;  (4)  unclamp,  and  with  the  edge  of  the  alidade 
ruler  passing  through  a  and  d,  turn  the  plane  table  to  such  a 
position  that  point  C  is  seen  through  the  telescope;  (5)  clamp, 
and  pivot  the  alidade  about  the  point  a,  sighting  point  B,  and 
draw  a  ray  intersecting  the  ray  drawn  from  c  at  some  point  x ; 
(6)  unclamp,  and  with  the  edge  of  the  alidade  ruler  through 
points  b  and  x,  turn  the  plane  table  to  such  a  position  that 
point  B  may  be  seen  through  the  telescope.  The  plane  table  is 
then  correctly  oriented.  As  a  check  azimuth  rays  may  be  drawn 
from  the  three  visible  points  back  through  their  plotted  points : 
these  rays  should  intersect  in  the  same  point. 

The  Three-Point  Problem,  Continued:  (3)  By  Mechanical 
Orientation. 

Set  up,  level,  and  attach  a  piece  of  tracing  paper  on  the  plane 
table.  Assume  an  arbitrary  point  x  on  the  tracing  paper,  and 
on  this  point  pivot  the  alidade  ruler;  sight  the  three  visible 
points,  A,  B,  and  O,  in  succession,  drawing  light  pencil  rays 
each  time.  These  rays  will  all  intersect  at  x.  Now  loosen  the 
tracing  paper  ABD  and  so  adjust  it  on  the  field  sheet  that  the 
rays  will  exactly  coincide  with  their  plotted  points  a,  b,  and  c : 
x  on  the  tracing  paper  will  then  be  over  its  true  position  on 
the  field  sheet.  Mark  x  on  the  field  sheet  by  pricking  with  a 
sharp  pointed  pencil.  With  the  edge  of  the  alidade  ruler 
through  points  x  and  a,  turn  the  plane  table  so  that  A  can  be 
seen  through  the  telescope;  clamp.  The  plane  table  is  then 
correctly  oriented. 

In  addition  to  the  Two-  and  the  Three-Point  Problems,  there 
are  two  modifications  that  are  often  useful,  especially  in  "filling- 
in"  work. 


§ 


.§ 


Military  Topography  and  Photography 


75 


RANGING  IN :  Having  given  a  direction  line  and  one  visible 
plotted  point  outside  of  that  Iwe,  to  determine  the  map  position 
of  an  unknown  point  X  on  that  line.  Fig.  26. 

The  direction  line  may  be  a  plotted  road,  railroad,  stream, 
fence,  ridge,  or  other  like  object.  Set  up  on  the  unknown  sta- 
tion X,  level  the  plane  table,  and  then  orient  by  placing  alidade 
along  the  plotted  direction  line  and  turning  the  plane  table  to 
such  a  position  that  the  alidade  telescope  is  sighted  on  the 
actual  direction  line.  Now  pivot  the  alidade  about  the  visible 

f  *     . 

v 
\ 

\ 

\ 


\  I 


FIG.  26 — "RANGING  IN" 

-  ^r 

plotted  point  a  until  point  A  is  sighted;  draw  a  ray  until  it 
intersects  the  given  plotted  direction  line.  This  "intersection  is 
the  true  map  position  of  the  unknown  point  X.  Where  a  ridge 
or  other  broad  line  is  used  as  the  direction  line,  the  result  will  of 
course  not  be  so  accurate. 

LINING  IN:  Having  given  only  one  visible  plotted  point  a, 
to  determine  the  map  position  of  an  unknown  point  X  by  means 
of  a  subsequently  determined  instrument  station.  Fig.  27. 

This  method  may  be  employed  where  only  one  visible  plotted 
point  is  visible,  the  other  visible  terrain  being  unknown  (un- 


76 


Military  Topography  and  Photography 


plotted) .  Set  up  at  the  unknown  station  X,  level,  and  orient  by 
the  compass;  pivot  the  alidade  on  the  visible  plotted  point  a 
and  sight  A ;  draw  a  ray  indefinitely ;  mark  the  station  on  the 
ground  with  a  stake.  Now  occupy  another  unknown  point  Y, 
visible  to  X  and  also  to  two  or  more  visible  plotted  stations,  and 
determine  the  map  position  y  of  Y  by  any  of  the  methods  ex- 
plained above.  Pivot  the  alidade  ruler  on  y,  and  sighting  X 
draw  a  pencil  ray  to  intersect  the  ray  ax.  This  intersection  is 
the  map  position  of  point  X. 


1 


FIG.  27— "LINING  IN" 

The  following  method  may  also  be  used,  but  if  the  angle 
formed  at  the  known  visible  plotted  point  is  less  than  30°,  the 
method  should  be  used  only  in  rapid  exploratory  surveys. 

THE  ONE-POINT  PROBLEM:  Having  given  a  large  area 
in  which  only  one  plotted  point  is  visible,  to  determine  the  map 
position  of  an  unknown  point  X  within  that  area.  Fig.  28. 

Set  up  at  the  unknown  point  X,  level,  and  orient  by  the  com- 
pass ;  pivot  the  alidade  ruler  on  the  plotted  point  a,  and  sighting 


1**  faith* 


FIG.  28— ONE-POINT  PROBLEM 


78  Military  Topography  and  Photography 

A  draw  a  pencil  ray  ;  select  an  arbitrary  point  x^  on  the  ray  ax 
as  the  present  location  of  x  ;  pivot  the  alidade  ruler  on  xx,  and 
sighting  on  an  auxiliary  point  Y,  at  about  right  angle  to  A  and 
at  such  a  distance  from  X  that  an  angle  of  at  least  30°  is  formed 
at  A  in  the  angle  XAY,  draw  a  pencil  ray  calling  it  Xiji  ; 
measure  the  distance  XY  ;  set  the  plane  table  up  at  Y  and  orient 
it  by  placing  the  alidade  ruler  along  the  ray  Xiji,  and  turning 
the  board  so  that  X  is  sighted  ;  then  pivot  the  alidade  ruler  on 
a,  and  sighting  A,  draw  a  pencil  ray  intersecting  the  ray  Xiya  ; 
measure  the  angles  axiji  and  ayxx!  with  a  protractor.  We 
now  know  the  three  angles  of  the  triangle  XAY  and  the  length 
of  one  side,  from  which  the  other  two  sides  may  be  computed 
by  the  following  trigonometric  formula: 

Z  XAY  =  180°  —  (Z  AXY  +  Z  AYX) 
Sin  AYX 

AX  ~~  Sin  XAY 
Sin  AXY 


Measure  off  the  distance  AX  on  the  ray  ax:  from  a,  the 
plotted  point  x  is  the  map  position  of  the*  unknown  point  X. 
Point  Y  may  be  similarly  determined.  The  measurement  of 
these  angles  with  a  protractor  is  not  very  accurate  :  if  the 
angles  are  measured  with  a  transit  directly,  this  method  is  fairly 
accurate  when  the  angle  at  A  is  30°  or  over.  If  x^i  is  plotted 
to  scale  from  the  measured  distance,  XY,  the  map  position  of 
x  and  y  can  be  found  by  constructing  the  parallelogram  Xijiyx, 
the  point  y  being  determined  by  the  intersection  of  Aa  with  yiy, 
as  shown  in  Fig.  28. 

(2)  Location  by  Meaiidation:  Whcnevej  a  traverse  is  made 
in  topographic  surveys,  the  sketching  of  the  terrain  along  the 
route  of  the  traverse  is  always  done.  This  sketching  will  extend 
from  about  one-quarter  to  one-half  mile  to  cither  side  of  the 
traverse:  the  traverse  stations  thus  serve  as  topographic 
instrument  stations  too. 

Meandation,  or  traversing,  will  be  used  when  the  ground  is  so 
level  or  so  wooded  as  to  render  resection  work  impracticable. 
For  primary  control  in  mcandation,  control  traverses  are  run  ; 


Military  Topography  and  Photography  79 

for  secondary  control,  needle  and  back-sight  traverses— ^needle 
traverses  when  the  magnetic  needle  can  be  depended  upon,  and 
back-sight  traverses  when  it  cannot.  In  the  execution  of  control 
traverses,  the  character  of  the  work  required  and  the  terrain  will 
determine  the  areas  of  enclosure.  Where  long  control  traverses 
are  run,  the  azimuth  should  be  checked  daily  by  sun  azimuth 
and  a  difference  over  one  minute  for  15  set-ups  should  cause 
the  rejection  of  the  work  completed  since  the  preceding  sun 
azimuth.  The  error  of  enclosure  should  not  exceed  one  minute 
for  fifteen  set-ups,  or  stations,  or  about  .5'  X  VN,  where  N  is 
the  number  of  set-ups ;  and  in  lineal  measurement  the  error 
should  not  exceed  1  in  500. 

CRITICAL  POINTS.  A  critical  point  of  the  terrain  is  a  loca- 
tion on  it  where  there  is  an  abrupt  change  in  the  slope  or  in 
direction.  Such  points  are  the  top,  the  military  crest,  and  the 
foot  of  hills ;  bends  in  streams  and  roads ;  road  crossings ;  etc. 
There  is  an  infinite  number  of  critical  points  except  on  the  most 
level  of  terrain,  but  for  practical  work  it  is  only  possible  to 
determine  geometrically  the  most  important  or  control  critical 
points.  The  intermediate  points  are  determined  by  estimation, 
aiding  and  checking  the  estimates  by  comparison  with  the  con- 
trol critical  points  geometrically  determined.  Herein  the  skill 
and  the  judgment  of  the  topographer  are  required — to  know 
what  points  to  measure,  what  points  to  estimate,  and  what 
points  to  ignore,  in  order  to  interpret  them  into  a  faithful  and 
intelligent  representation  of  the  terrain. 

In  topographic  surveys,  critical  points  are  determined  geo- 
metrically in  four  ways — by  radiation,  intersection,  resection, 
and  meandation. 

(1)  Location  by  Radiation:  In  this  method  the  critical 
points  within  range  of  an  instrument  station  are  determined 
by  plotting  the  direction,  measuring  the  distance  with  tape  or 
stadia,  and  determining  the  vertical  angles  of  those  points,  all 
from  that  instrument  station.  With  the  plane  table  proceed 
as  follows:  (1)  set  up  and  level  the  plane  table  at  the  instru- 
ment station;  (2)  orient  the  plane  table  and  locate  the  station 
by  any  method  explained  in  the  two  preceding  paragraphs, 
plotting  the  station  as  instrument  station  number  one,  abbre- 


80  Military  Topography  and  Photography 

viated  "1-1";  send  a  rodman  to  hold  the  stadia  rod  on  the  first 
critical  point,  a;  pivot  the  alidade  about  the  plotted  point 
"1-1,"  sight  the  stadia  rod,  draw  a  pencil  ray  to  represent  its 
azimuth  on  the  field  sheet;  read  the  stadia  intercept  on  the 
stadia  rod,  and  record  the  vertical  angle;  the  stadia  reading 
corrected  for  the  horizontal  (stadia  reading  X  the  cosine2  of  the 
vertical  angle)  gives  the  distance  to  measure  off  on  the  azimuth, 
I-la,  to  determine  on  it  the  map  position  of  the  critical  point  a. 
Similarly,  determine  all  the  critical  points  around  the  instru- 
ment station  and  plot  them  on  the  field  sheet.  The  determina- 
tion of  the  elevation  of  critical  points  and  the  sketching  of  the 
terrain  will  be  taken  up  later. 

(2)  Location  by  Intersection:  Having  given  two  instru- 
ment stations,  1-1  and  I-£,  to  determine  the  map  position  of  a 
critical  point  X  which  is  visible  to  both  instrument  stations. 

Intersection  as  here  applied  is  the  determination  of  the  loca- 
tion of  a  critical  point  by  the  intersection  of  azimuth  rays  to  it 
from  two  or  more  instrument  stations.  Procedure:  (1)  Set 
up,  level,  orient,  and  locate  the  map  position  of  the  plane  table 
at  "1-1";  (2)  pivot  the  alidade  on  "1-1,"  sight  X  and  draw  a 
pencil  ray  indefinitely  (whenever  a  sight  is  taken  on  a  critical 
point,  its  vertical  angle  is  always  read)  ;  (3)  set  up,  level, 
orient,  and  locate  the  map  position  of  the  plane  table  at  "1-2" ; 
(4)  now  pivoting  the  alidade  on  the  plotted  point  "1-2,"  sight 
X  and  draw  a  pencil  ray  indefinitely :  the  intersection  of  these 
two  plotted  rays  is  the  map  position  of  the  critical  point  X. 

Intersection  dispenses  with  stadia  men,  but  its  use  -is  more 
limited  than  that  of  radiation,  for  the  points  selected  for  inter- 
section must  be  visible  from  two  instrument  stations  and  the 
angle  of  intersection  should  not  be  less  than  30°  nor  more  than 
120°.  A  combination  of  both  methods  is  of  course  the  best. 
While  locating  the  near  accessible  points  by  stadia  "readings, 
the  more  distant  and  inaccessible  points  can  be  determined  by 
intersection.  At  each  instrument  station  the  topographer  will 
take  azimuth  rays  on  all  points  that  he  wishes  to  determine  by 
intersection;  such  points  must  be  so  named  or  described  that 
they  may  be  identified  or  recognized  at  some  other  instrument 
station.  Objects  appear  different  when  viewed  from  different 


Military  Topography  and  Photography 


81 


positions  and  it  is  also  very  difficult  to  identify  stations  when  a 
large  number  remain  unintersected  at  the  same  time. 

(3)  Resection  and  Meandation:  Critical  Points  so  deter- 
mined are  occupied  the  same  as  Instrument  Stations. 

ELEVATION  OF  INSTRUMENT  STATIONS  AND  CRITICAL,  POINTS. 
In  topographic  surveying  the  elevation  of  unknown  points  is 
determined  from  the  difference  in  elevation  between  them  and 


COX'S   STADIA   COMPUTER. 


pposue  me  vertical  angle  of  the  transit  telescope  find 
be  DifTereuce  of  Elevation,  and  opposite  the  same  angle 
the  Distance  Scale  rtud  the  Horizontal  Distance. 


EXAMPLE. 

Vertical  angle  12'  30'.  reading  of  the  Rod  537  feet.    Bet  the 

icro  of  the  disc  opposite  W7.  and  opposite  12'  30-  of 

tlic  scale  at  the  left  read  113}  feet  Difference  of 

Elevation,  and  opposite  12"  30',  of  the  Scale 

at  the  right  read  412  feet  Distance. 


Copyright.    1899.    by 


Designed  by  Wm.  Coi. 


*FOR  REDUCING  INCLINED  STADIA  READING 

known  stations.  The  difference  in  elevation  between  two  points 
is  equal  to  either,  (1)  the  slope  distance  between  them  times 
the  sine  of  their  vertical  angle,  or  (2)  the  horizontal  distance 
between  them  times  the  tangent  of  their  vertical  angle.  If  the 
vertical  angle  is  less  than  5°,  the  sine  may  be  used  when  only 
the  horizontal  distance  is  known.  To  use  Cox's  Stadia  Com- 
puter to  solve  elevation  problems:  (1)  when  the  slope  distance 
and  vertical  angle  are  known — set  the  "0"  or  index  of  the  inner 


*  Courtesy  of  W.  &  L.  E.  Gurley. 


82 


Military  Topography  and  Photography 


circle  into  coincidence  with  the  number  representing  the  slope 
distance  on  the  outer  circle ;  the  difference  in  elevation  will  be 
that  number  on  the  outer  circle  which  is  in  coincidence  with  the 
vertical  angle  as  read  on  that  portion  of  the  inner  circle  labeled 
"Difference  in  Elevation";  (2)  when  the  horizontal  distance  is 
known  and  the  vertical  angle  is  greater  than  5° — set  the  num- 
ber representing  the  vertical  angle  on  that  portion  of  the  inner 


*BEAMAN  STADIA   ARC 
FOR  MECHANICALLY  REDUCING  INCLINED  STADIA  READINGS 

circle  labeled  "Hor.  Distance"  into  coincidence  with  that  num- 
ber on  the  outer  circle  representing  the  horizontal  distance, 
read  for  difference  in  elevation  as  in  (1). 

(1)  Elevation  in  Resection:  Point  12  in  Fig.  33  has  been 
determined  by  resection  on  triangulation  stations,  A,  B,  and  C ; 
the  vertical  angle  from  12  to  AA  is  +  30' ;  from  12  to  AB, 

*  Courtesy  of  W.  &  L.  E.  Gurley. 


Military  Topography  and  Photography  83 

+  54';  and  from  12  to  AC,  +  58'.  The  elevation  of  A  A  is  1175 
feet,  of  AB  1268  feet,  and  of  AC  1226.  To  determine  the  eleva- 
tion of  12,  set  the  points  of  a  pair  of  dividers  to  coincide  with 
points  AA  and  12 ;  then  apply  the  points  of  the  dividers  to  a 
reading  scale  and  read  the  intercept  in  feet,  which  is  the  ground 
distance  from  12  to  AA:  similarly  measure  the  map  distances 
of  12  to  AB  and  of  12  to  AC,  respectively ;  with  these  distances 
as  the  horizontal  distances  and  the  vertical  angles  as  read,  solve 
the  difference  in  elevation,  using  either  the  trigonometric 
formula,  or  Cox's  Stadia  Computer.  These  differences  sub- 
tracted from  the  respective  elevations  of  AA,  AB,  and  AC,  should 
all  give  the  same  value  for  the  elevation  of  12.  (In  long  distances, 
correction  should  be  made  for  the  curvature  of  the  earth.) 
Where  there  is  a  slight  variation  take  the  mean  as  the  most 
probable  value.  Usually  the  determination  of  elevation  from 
the  two  outer  triangulation  stations  is  sufficient,  but  if  the  two 
values  vary  too  much,  the  elevation  from  the  third  triangulation 
station  should  be  taken  as  a  check  and  to  indicate  the  more 
probable  correct  elevation  from  the  first  two  stations. 

(2)  Elevation  in  Meandation:   The  elevation  of  all  instru- 
ment stations  in  traverses  is  determined  and  recorded.     The 
distance  between  successive  stations  is  measured  by  stadia  or 
tape,  the  vertical  angle  is  read  from  the  transit  or  alidade  tele- 
scope vernier.     When  distance  is  measured  with  the  stadia  jthe 
slope  distance  is  obtained;  when  it  is  measured  with  a  tape  or 
chain,  the  horizontal  distance  is  obtained.     The  difference  in 
elevation   between  successive   stations   is   added   or   subtracted 
according  as  to  whether  it  is  plus  or  minus,  and  the  elevation 
is  thus  carried  forward. 

(3)  Elevation  in  Radiation:     One  half  the  stadia  reading 
from  an  instrument  station  to  a  critical  point  times  the  sine 
of  twice  the  vertical  angle  gives  the  difference  in  elevation  be- 
tween them;  this  difference  in  elevation  added  to  or  subtracted 
from  the  elevation  of  the  instrument  station,  according  as  to 
whether  the  vertical  angle  is  plus  or  minus  gives  the  elevation 
of  the  critical  point. 

(4)  Elevation  in  Intersection:     This  method  is  just  the 
same  as  in  resection.     Measure  with  the  dividers  the  distance 


84  Military  Topography  and  Photography 

from  the  first  instrument  station  to  the  intersection  or  critical 
point;  similarly  measure  the  distance  from  the  second  instru- 
ment station  to  the  critical  point ;  then  solve  for  the  differences 
in  elevation,  and  add  to  or  subtract  from  the  respective  eleva- 
tion of  the  two  instrument  stations,  according  as  to  whether 
the  vertical  angle  is  plus  or  minus.  The  two  results  should  be 
the  same  or  nearly  the  same ;  take  the  mean  as  the  more  probable 
correct  elevation  of  the  critical  point. 

SKETCHING  OPERATIONS 

PLOTTING  DIRECTION.  Direction  as  explained  in  a  previous 
paragraph  is  the  azimuth  of  a  line.  In  sketching,  the  plotted 
azimuth  or  bearing  of  a  line  with  respect  to  the  field  sheet  must 
of  course  bear  the  same  relation  as  the  azimuth  or  bearing  of  a 
ground  line  with  respect  to  the  terrain.  In  topographic  sur- 
veying the  azimuth  of  a  line  is  always  determined,  so  in  the 
description  of  sketching  operations  the  azimuth  will  always  be 
used  in  this  book.  If  the  bearing  of  a  line  is  ever  given,  the 
topographer  should  change  it  to  its  azimuth.  There  are  several 
methods  by  which  the  azimuth  of  a  line  can  be  plotted  on  tht- 
field  sheet. 

(1 )  With  Alidade  Ruler:  This  is  the  method  used  with  the 
plane  table.  It  should  be  remembered  that  the  statement  "the 
plane  table  is  oriented"  means  that  the  field  sheet  on  the  plane 
table  is  oriented.  Having  given  the  plane  table  set  up,  leveled 
and  oriented  at  a  station  A,  to  plot  the  direction,  or  azimuth, 
to  a  visible  point  X.  Procedure :  ( 1 )  Pivot  the  edge  of  the 
alidade  ruler  about  the  plotted  point  a,  sticking  a  pin  in  point 
a  so  as  to  keep  the  alidade  ruler  in  contact  with  a ;  (2)  center  the 
telescope  on  the  visible  point  X;  (3)  draw  a  light  pencil  ray 
along  the  edge  of  the  alidade  ruler  indefinitely  from  a  towards 
the  visible  point  X;  this  plotted  pencil  ray  lies  in  the  same 
direction  on  the  field  sheet  as  the  direction  of  X  from  A  on  the 
ground. 

(%)  With  T-Square  and  Protractor:  This  method  is  em- 
ployed where  the  sketching  board  is  used ;  the  azimuth,  or  direc- 
tion, on  the  ground  being  determined  with  a  transit,  prismatic 
compass,  or  other  instrument.  Having  given  the  direction,  or 


Military  Topography  and  Photography  85 

azimuth,  of  a  line  AX  determined  with  a  transit  or  prismatic t 
compass,  to  plot  the  same  on  a  sketching  board  by  means  of  a 
T-Square  and  Protractor.  In  this  method  the  north  and  south 
line  on  the  field  sheet  must  be  perpendicular  to  the  left  edge  of 
the  sketching  board.  We  shall  assume  that  the  azimuth  from 
A  to  X  as  determined  to  be  210°.  Procedure:  (1)  Keeping  the 
plumb  edge  of  the  T-square  in  contact  with  the  left  edge  of  the 
sketching  board,  bring  the  straight  edge  of  the  T-square  in 
contact  with  the  plotted  point  a;  (2)  with  the  straight  edge 
of  the  protractor  in  contact  with  the  straight  edge  of  the 
T-square,  bring  the  center  *point  of  the  protractor  over  the 
plotted  point  a;  (3)  make  a  pencil  mark  at  210°  (or  30°); 
(4)  remove  the  protractor  and  T-square  and  through  the  plot- 
ted point  a  and  the  pencil  mark  draw  a  pencil  ray  indefinitely: 
the  plotted  line  is  the  azimuth  ray  ax.  It  is  assumed  that  either 
the  rectangular  or  semi-protractor  is  used;  the  protractor 
should  have  two  series  of  numbers  on  the  scale,  one  from  0°  to 
180°  and  the  other  from  180°  to  360°. 

(3)  With  Paper  Protractor  and  Triangles:  This  method 
is  used  where  tracing  paper  or  linen  fastened  over  a  paper  pro- 
tractor is  employed  for  the  field  sheet:  the  sketching  board  is 
not  oriented.  Having  given  the  azimuth  of  a  line  AX  de- 
termined with  a  transit  or  prismatic  compass,  to  plot  the  same 
on  the  fteld  sheet.  We  shall  assume  the  azimuth  as  determined 
to  be  165°.  Procedure:  (1)  Place  one  triangle  so  that  its 
hypotenuse  passes  through  the  center  point  of  the  protractor 
and  the  division  on  the  protractor  scale  representing  165°;  (2) 
holding  this  triangle  firmly  in  place,  place  a  second  triangle 
with  its  hypotenuse  in  contact  with  one  leg  of  the  first  triangle ; 
(3)  now  holding  the  second  triangle  firmly  in  place  with  one 
hand,  slide  the  first  triangle  along  the  hypotenuse  of  the  second 
triangle  by  means  of  the  free  hand  until  the  hypotenuse  of  that 
.(first)  triangle  comes  in  contact  with  the  plotted  point  a;  (4) 
then  hold  the  first  triangle  in  place  and  draw  a  pencil  ray  ax 
indefinitely  from  a  along  the  hypotenuse  of  this  triangle,  in  the 
same  direction  as  the  azimuth  reading  is  from  the  center  of  the 
protractor:  the  plotted  line  is  the  azimuth  ray  ax. 


86  Military  Topography  and  Photography 

PLOTTING  DISTANCE.  Distance  on  a  map  is  always  plotted 
to  some  given  scale.  This  scale,  or  ratio  of  map  distance  to 
ground  distance  has  been  discussed  in  a  previous  chapter.  Dis- 
tance can  be  plotted  or  measured  off  in  several  ways. 

( 1 )  With  Reading  Scale:  Having  given  a  plotted  azimuth 
ax^  the  ground  distance  AX,  to  plot  the  map  position  x  of  the 
visible  point  X.  Procedure:  (1)  place  the  edge  of  the  reading 
scale  along  the  plotted  azimuth  .ax1?  with  its  zero  division  on 
the  plotted  point  a;  (2)  at  the  division  on  the  reading  scale 
representing  the  ground  distance  AX  make  a  pencil  mark ;  this 
mark  on  axx  is  the  map  position  x  of  the  visible  point  X. 

(*2)  With  Strip  of  Blank  Paper:  Having  given  a  plotted 
azimuth  ax^  and  the  ground  distance  AX,  to  plot  the  map  posi- 
tion x  of  the  visible  "point  X.  This  method  is  used  where  it  is 
not  possible  to  apply  the  reading  scale  directly.  Procedure: 
(1)  Place  the  edge  of  the  strip  of  blank  paper  along  the  edge 
of  a  reading  scale  and  mark  on  it  a  short  line  opposite  the  zero  of 
the  reading  scale  and  another  mark  opposite  the  division  on 
the  scale  which  represents  the  ground  distance  AX:  (2)  place 
the  strip  of  paper  along  the  ray  axi  with  the  first  mark  at  the 
plotted  point  a  and  make  a  pencil  mark  on  ax!  opposite  the 
second  mark  on  the  strip  of  paper :  this  pencil  mark  is  the  map 
position  x  of  the  visible  point  X. 

(3)  With  Dividers:  Having  given  a  plotted  azimuth  ax^ 
and  the  ground  distance  AX,  to  plot  the  map  position  x  of  the 
visible  point  X.  This  method  is  used  where  the  reading  scale 
is  not  available  for  using  directly :  this  will  usually  be  the  con- 
dition in  field  work  where  the  reading  scale  is  fastened  on,  or 
drafted  on  the  sketching  board.  Procedure:  (1)  Open  the 
dividers  so  that  when  one  point  is  on  the  zero  of  the  reading 
scale,  the  other  point  is  at  that  division  of  the  scale  which  repre- 
sents the  ground  distance  AX;  (2)  now  apply  one  point  of  the 
dividers  to  the  plotted  point  a,  and  with  the  other  point  inter- 
cept the  azimuth  axx,  making  a  small  pin  hole :  this  jiin  hole  is 
the  map  position  x  of  the  visible  point  X. 

In  this  section  we  have  used  the  reading  scale  as  a  working 
scale,  for  our  working  units  in  topographic  surveying  are  the 
foot,  mile,  etc.  In  rapid  sketching  we  shalj  use  the  stride,  pace, 


88  Military  Topography  and  Photography 

etc.,  as  our  working  unit,  and  our  scale  will  be  a  true  working 
scale  according  to  our  definition.  It  is  better  to  call  a  working 
scale  in  feet  or  meters  a/  reading  scale,  even  though  at  variance 
with  our  definition. 

PLOTTING  SLOPES.  All  slopes  may  be  divided  into  three 
general  classes:  (1)  even,  (2)  convex,  and  (3)  concave;  the 
terrain  is  made  up  of  a  succession  of  these  slopes  and  the  divid- 
ing lines  between  these  slopes  are  the  controlling  lines  of  the 
terrain.  Since  lines  are  determined  by  points,  it  is  only  neces- 
sary to  determine  the  critical  points  of  these  controlling  lines 
and  we  shall  have  a  complete  control  over  the  whole  area  inso- 
far as  the  conformation  of  the  ground  is  concerned.  The  prob- 
lem then  reduces  itself  to  plotting  the  simple  slopes  between 
these  points,  so  spacing  the  contour  lines  as  to  produce  an 
even,  convex,  or  concave  slope  of  the  degree  desired.  It  is  im- 
practicable to  run  contour  levels  and  traverses  to  locate  each 
contour  line;  their  spacing  must  be  done  by  estimation,  and 
with  practice  the  topographer  should  soon  be  able  to  space  and 
locate  contour  lines  with  a  high  degree  of  accuracy.  It  should 
be  remembered  that  a  faithful  representation  and  not  an  exact 
reproduction  of  the  terrain  is  desired ;  the  essential  details  must 
be  utilized  so  as  to  bring  out  the  predominant  features  of  the 
terrain.  Often  by  slightly  changing  the  horizontal  position  of 
a  portion  of  a  contour  line,  a  more  accurate  representation  of 
the  character  of  the  fclope  is  secured.  This  may  be  shown  in 
the  diagram,  Fig.  30b,  in  which  AB  is  a  profile  of  a  slope  in 
which  x,  a  critical  point,  has  an  elevation  of  83  feet,  while  the 
80  foot  elevation  is  at  y,  a  position  considerably  to  the  left. 
Now  if  the  80  foot  contour  were  to  be  drawn  along  the  80  foot 
elevation  line  at  this  place  a  map  as  shown  in  Fig.  30a,  would 
be  produced  and  the  inference  in  map  reading  would  be  that 
the  slope  between  the  80  and  100  foot  contours  was  an  easy 
slope  connecting  regularly  with  the  slopes  between  the  60  and 
80  foot  contours  and  the  100  and  120  foot  contours;  but  if 
the  80  foot  contour  is  made  to  pass  through  point  x,  the  map  as 
shown  in  Fig.  30c,  is  produced,  and  while  the  slope  xz  is  shown  a 
little  longer  than  "it  really  is,  its  degree  or  steepness  is  shown 
which  is  far  more  important  than  any  harm  from  a  slight 


Military  Topography  and  Photography 


89 


FIG.  30 

•  •  ••-•* 

variation  in  the  horizontal  location  of  a  portion  of  a  contour 
line.  To  what  extent  the  location  of  a  contour  may  be  changed 
in  order  to  represent  more  accurately  the  character  of  the 
terrain  can  hardly  be  stated.  The  experienced  topographer  will 
know  from  the  importance  of  that  portion  of  the  terrain  which 
he  wishes  to  represent.  Often  an  intermediate  dotted  contour 
line  can  be  very  advantageously  used. 

(1)  Even  Slopes:  In  Fig.  3  are  drawings  showing  the 
spacing  of  contours  for  even  slopes  of  different  degrees;  with 
practice  the  topographer  will  be  able  to  space  contours  by  esti- 
mation, but  beginners  will  have  to  use  the  slope  scale.  A  con- 
crete example  of  contour  spacing  will  be  given.  Having  given 
two  plotted  critical  points,  a  and  b,  elevation  75  and  170  feet, 
respectively,  with  an  even  slope  between,  to  plot  the  contours; 
V.  I.  %0  feet.  Procedure:  Between  points,  a  and  b,  the  80, 
100,  120,  140,  and  160  contours  must  be  drawn;  from  a  to  the 
80  foot  contour  is  I/A  of  a  contour  interval,  from  the  80th  to 


90  Military  Topography  and  Photography 

the  160th  foot  contours  there  are  4  contour  intervals,  and  from 
the  160  foot  contour  to  b  is  %  of  a  contour  interval — in  all, 
4%  contour  intervals.  The  distance  ab  must  therefore  be 
divided  into  4%  intervals.  The  80  foot  contour  is  placed  %  of 
an  interval  from  a  towards  b,  the  160  foot  contour  is  placed  % 
of  an  interval  from  b  towards  a,  and  the  space  between  the  80  and 
160  foot  contours  is  divided  into  four  equal  spaces  for  the  three 
contours— the  100  foot,  the  120  foot,  and  the  140  foot.  Fig.  31, 


vl 


FIG.  31 


FIG.  32 

Having  given  two  plotted  critical  points,  a  and  b,  the  eleva- 
tion of  a  90  feet,  and  an  even  slope  of  2°  between  them,  to  plot 
the  contours.  Procedure:  (1)  Place  the  edge  of  the  2°  slope 
scale  through  the  plotted  points  a  and  b  so  that  point  a  is  mid- 
way between  two  divisions;  (2)  make  a  pencil  dot  on  the  field 
sheet  directly  next  to  each  division  on  the  slope  scale  between 
points  a  and  b:  these  pencil  dots  are  the  locations  of  all  the  con- 
tours between  a  and  b.  Fig.  32. 

(2)  Convex  Slopes:  For  convex  slopes  the  contours  arc 
nearer  together  at  the  bottom  of  the'  slope  and  farther  apart  at 
the  top.  The  spacing  must  be  such  that  the  proper  convexity  is 


Military  Topography  and  Photography  91 

shown;  and  the  difference  in  elevation  between  the  two  critical 
points  must  be  represented  by  the  proper  number  of  contour 
intervals. 

(3)  Concave  Slopes:     For  concave  slopes  the  contours  arc 
farther  apart  at  the  bottom  of  the  slope  and  closer  together  at 
the  top.     As  with  convex  slopes,  the  spacing  of  the  contours 
must  be  such  that  the  proper  degree  of  concavity  is  shown,  and 
the  difference  in  elevation  between  the  two  critical  points  must 
be  represented  by  the  proper  number  of  contour  intervals. 

(4)  Changes  between  Slopes:     Usually  where  the  contours 
of  two  adjacent  slopes  have  been  plotted,  no  special  notice  need 
be  taken  of  the  point  of  juncture,  or  critical  point,  but  where 
such  critical  point  is   an  essential  part  of  the  terrain  to  be 
shown,  the  two  including  contours  should  be  so  plotted  as  to 
show  the  proper  compound  curvature,  even  though  it  is  neces- 
sary to  change  slightly  the  horizontal  position  of  a  contour,  as 
was  shown  in  the  beginning  of  this  section.       / 

PLOTTING    CHARACTER    OF    THE,    TERRAIN.       (1)    Natural 
Character  of  the  Terrain. 

(a)  Planes:      The    critical    points    of    level    ground    and 
ground  of  even  slopes  are  the  limiting  points  of  such  areas.   "On 
large  planes  such  points  will  usually  have  to  be  determined  by 
traverses ;  these,  with  such  other  critical  points  that  are  deter- 
mined to  plot  streams,  roads,  etc.,  will  make  the  plotting  of 
contours  for  planes  an  easy  task. 

(b)  Hills  and  Valleys:     There  is  always  a  valley  or  plane 
on  each  side  of  a  hill,  and  a  hill  on  each  side  of  a  valley.     Their 
line  of  juncture  in  each  case  is,  therefore,  a  control  line  whose 
critical  points  are  common  to  each.     These  critical  points  with 
those  which  control  the  bottom  of  valleys  and  the  top  of  hills, 
are  the  points  essential  to  the  plotting  of  hilly  terrain.     The 
hill  between  two  valleys  is  called  a  "water  shed,"  and  this  water 
shed  may  be  a  knoll,  a  well-defined  ridge,  or  a  broad  table-land. 
In  any  vertical  plane  of  a  hill  the  critical  points  are  the  top, 
the  military  crest,  and  the  foot  of  the  same  (its  junction  with 
the  adjacent  valleys)  ;   in    any  horizontal  plane   of  a  hill  the 
critical  points  are  the  ends,  the  sides,  changes  in  direction,  and 
junction  with  other  hills  or  ridges;  in  any  vertical  plane  of  a 


92  Military  Topography  and  Photography 

valley  the  critical  points  are  the  bottom  of  the  valley  and  its 
junctures  with  the  adjacent  hills;  in  any  horizontal  plane  of  a 
valley  the  critical  points  are  the  head,  the  mouth,  the  sides,  and 
the  changes  in  direction  of  the  valley.  All  important  streams 
will  be  traversed  which  will  give  the  control  for  the  elevation 
and  horizontal  position  for  the  bottom  of  their  valleys. 

(c)  Mountains:     The  critical  points  of  mountains  are  the 
same  as  those  of  hills,  but  are  much  more  difficult  to  determine. 
The  ascent  and  descent  of  mountains  are  very  difficult  to  make 
so  that  the  number  of  instrument  stations  are  reduced  to  a 
minimum,  while  the  location  of  critical  points  will  usually  be 
made  by  intersection.     Many  details  on  large  mountains,  which 
on  level  terrain  would  be  essential  can  be  ignored.     Mountains 
over  two  thousand  feet  in  height  from  their  base  can  be  more 
rapidly  and  easily  surveyed  and  plotted  by  photo-topographic 
methods. 

(d)  Cliffs:     In  plotting  steep  cliffs  intermediate  contours 
are  omitted.    Thus  with  twenty  foot  contour  intervals  only  the 
hundred  foot  contours  are  plotted  through  the  length  of  the 
cliff.     Through  vertical  cliffs  all  contours  will  unite  in  one  line. 
In  overhanging  cliffs,  the  lower  contours  will  cross  the  higher 
contours  through  the  length  of  such  cliffs,  and  such  lower  con- 
tours are  dotted  through  the  length  which  they  cross  higher 
contours. 

(e)  Streams:     The    critical   points    of    a    stream   are   its 
source,  its  mouth,  and  its   changes  of  direction.     Where  the 
stream   is   fairly   straight   only   a   sufficient  number  of  points 
should   be   determined   to   insure   the    correct   plotting   of   the 
stream.      Usually    traverses    will    be    run    to    plot    important 
streams — control  traverses  for  the  more  important  and  needle — 
and  back-sight  traverses   for  the  less   important.      Sometimes 
rivers  may  be  more  rapidly  and  easily  surveyed  and  plotted 
by  determining  the  important  changes  in  direction  throughout 
its  course  by  resection.     In  such  cases  the  critical  points  are 
occupied  as  resection  instrument  stations,  or  resection  instru- 
ment stations  are  determined  on  the  adjacent  water  sheds  while 
the  critical  points  on  the  stream  are  determined  by  stadia  or 
intersection  from  these  stations  as  shown  in  the  diagram,  Fig.  33. 


Military  Topography  and  Photography  03 

In  many  cases  critical  points  along  a  stream  cannot  be  seen 
or  occupied,  as  where  the  stream  flows  through  a  swamp  or 
thicket.  In  such  cases  it  is  usually  possible  to  mark  critical 
points  along  the  stream  by  erecting  at  such  points  flag  poles 
of  such  height  with  white  streamers  that  they  may  be  seen  and 
intersected  on  from  resection  instrument  stations  on  the  ad- 
jacent highlands.  With  large  scale  maps  the  width  of  rivers 
must  be  determined  and  plotted. 

(f)  Lakes  and  Swamps :  The  shores  of  lakes  and  the  edges 
of  swamps  are  determined  and  plotted  the  same  as  the  courses  of 
streams.  Their  plotting  presents  no  special  difficulty. 

(%)     Vegetation: 

(a)  Grass:     The  area  and  extent  of  grass  land  is  deter- 
mined and  plotted  with  the  proper  conventional  symbol.    Where 
the  grass  is  low,  as  in  a  pasture,  all  topographic  operations 
are  easy,  but  where  the  grass  is  very  high  control  work  is  exceed- 
ingly difficult. 

(b)  Forests:    The  limits  of  forest  lands  are  determined  and 
plotted,  but  the  conformation  of  the  ground  in  such  areas  is 
plotted  only  in  a  broad  generalization.    Usually,  and  especially 
in  forests  on  level  ground,  the  control  will  have  to  be  made  by 
traverses.    Where  a  few  high  and  open  hills  overlook  the  woods, 
triangulation  can  be  carried  on  to  a  considerable  extent.    Criti- 
cal points  throughout  the  woods  may  be  marked  by  fastening  a 
flag  pole  and  streamer  to  a  high  tree  at  such  points,  so  that  the 
streamers  can  be  seen  from  the  high  knolls  where  such  streamers 
can  be  located  by  intersection.     Often,  too,  from  an  open  space 
or  a  high  tree,  in  such  woods,  the  topographer  can  locate  him- 
self by  resection.     Topographic  details  of  the  ground  which 
are  very  essential  in  the  open,  are  unimportant  in  a  map  of  the 
woods. 

(c)  Cultivated  Fields:  Cultivated  fields  are  plotted  as  such, 
but  the  character  of  vegetation  is  not  shown  unless  the  map 
is  for  immediate  use,  or  the  vegetation  is  permanent  as  in  rice 
fields,  cane  fields,  orchards,  etc. 

(3)     Lines  of  Communication: 

(a)  Roads  and  Trails :  Roads  are  determined  by  traverses 
or  by  triangulation  at  their  crossings,  forks,  bends,  etc.  Im- 


94  Military  Topography  and  Photography 

portant  trails  are  likewise  determined.     The  width  of  roads  is 
conventional  except  on  plots  of  large  scale. 

(b)  Railroads  and  Canals:     Railroads  and  canals  are  de- 
termined  in  the   same  manner   as   roads.      Side   tracks,   canal 
locks,  shops,  etc.,  are  also  very  important  on  military  maps. 

(c)  Bridges,  Ferries,  and  Fords:     The  kind,  width,  length, 
and  height  above  water  of  bridges ;  kinds  and  number  of  piers 
and  abutments  ;  kind,  depth  and  length  of  ferries  ;  and  the  width, 
depth  and  kind  of  bottom  of  fords,  are  determined  and  plotted. 

(4  )     Buildings  : 

(a)  Farm  Buildings:     Dwelling  houses  on  farms  are  de- 
termined and  plotted  with  the  conventional  sign ;  outbuildings 
are  not  plotted;  detached  barns  are.     The  name  of  the  owner 
is  also  plotted. 

(b)  Churdhes  and  School  Houses:     Churches   and  school 
houses  are  always  very  important  in  map  reading  as  points  of 
reference,  and  should  be  plotted  with  their  name. 

(c)  Towns  and  Cities:     The  solid  built  sections  of  cities 
are  plotted  in  solid  blocks  on  map,  dwelling  houses  in  the  thickly 
settled  sections  are  plotted  conventionally,  scattered  dwelling 
houses  are  each  plotted.     The  streets  of  cities  and  towns  are 
plotted. 

In  small  scale  maps,  cities  are  plotted  by  shaded  areas  while 
small  towns  are  represented  by  a  circle  (°). 

APPLIED  SKETCHING.  In  the  diagram,  Fig.  33,  is  shown  a 
portion  of  a  field  sheet.  The  field  sheet  on  the  plane  table 
would  be  about  the  same  size  as  the  board,  as  22'x28',  and  on 
it  would  be  plotted  all  primary  and  secondary  triangulation 
stations  within  the  area  which  the  field  sheet  represented.  In 
that  portion  of  the  field  sheet  shown  in  Fig.  33,  only  one 
triangulation  station  happens  to  lie. 

Designation  of  Stations:  Some  system  of  designation  should 
be  used  in  which  the  forms  suggest  the  character  of  the  stations. 
In  extended  surveys  primary  stations  should  be  designated  by 
proper  names — the  local  name  of  the  hill  on  which  the  station 
is  located;  if  there  is  no  local  name,  a  name  can  be  supplied. 
In  limited  surveys,  the  primary  stations  can  be  designated  by 


FIG.  33  (SCALK:  3"  =  1  MI.  V.I.  20') 


96  Military  Topography  and  Photography 

capital  letters  in  the  order  of  their  establishment ;  as,  AA,  AB, 
AC,  etc.  Secondary  stations  can  be  designated  numerically  in 
the  order  of  their  establishment;  as,  Al,  A2,  A3,  etc.  Tertiary, 
or  plane  table  resection  stations  can  be  designated  numerically 
in  the  order  of  their  establishment  preceded  by  the  letter  "I" ; 
as,  II,  12, 13,  etc.  The  instrument  stations  of  a  control  traverse 
can  be  designated  numerically  in  the  order  of  their  establish- 
ment preceded  by  the  letter  "T";  as,  TO,  Tl,  T2,  etc;  should 
a  traverse  start  from  a  triangulation  station,  the  designation 
of  such  station  will  be  used  in  lieu  of  "T-0."  Intersected 
points  can  be  designated  numerically  in  order  of  their  establish- 
ment without  reference  to  the  instrument  station  from  which 
they  are  intersected;  as,  1,  2,  3,  etc.  The  critical  points  around 
an  instrument  station,  established  by  a  stadia  reading,  can  be 
designated  by  small  letters  in  the  order  of  their  establishment 
at  each  instrument  station. 

For  convenience  of  reference,  the  elevation  of  triangulation 
stations  should  be  shown  in  figures,  either  near  each  station 
or  in  order  on  the  margin  of  the  field  sheet. 

Procedure:  In  Fig.  33,  the  field  sheet  having  been  prepared 
with  coordinate  lines  and  all  primary  and  secondary  stations 
plotted,  the  sketcher  is  ready  to  proceed  to  the  sketching.  Arriv- 
ing at  the  south  central  portion  of  the  field  he  selects  the 
southern  knoll  as  the  first  instrument  station,  and  locates  it  by 
resecting  on  AA,  AB,  and  AF;  the  elevations  of  these  triangula- 
tion stations  are  known;  the  vertical  angles  to  them  from  the 
instrument  station  (II)  are  read;  the  ground  distances  to  them 
are  obtained  by  applying  a  graphic  reading  scale  to  the  map 
distances — from  which  the  difference  in  elevation  is  obtained. 
These  differences  subtracted  from  the  respective  elevations  of 
the  triangulation  station  should  all  give  the  elevation  of  II, 
flrhich  results  should  check  within  a  few  feet.  The  elevation  of 
II  is  thus  found  to  be  125'.  Standia  readings  are  taken  to  the 
critical  points  a,  &,  c,  d,  e,  f,  g,  and  h.  By  the  use  of  a  stadia 
reduction  table  or  stadia  computer,  the  horizontal  distances  of 
and  differences  in  elevation  to  these  critical  points  are  obtained 
and  plotted.  With  this  data  the  sketcher  can  plot  the  hill  as 
far  north  as  the  top  of  the  spur  to  the  northeast.  The  sketcher 


Military  Topography  and  Photography  97 

in  his  preliminary  reconnaissance  observed  that  the  river  here 
flowed  through  an  impenetrable  swamp,  and  the  vegetation  was 
so  high  that  the  bends  of  the  river  through  the  swamp  could  not 
be  located  by  resection  from  those  bends  for  the  triangulation 
stations  could  not  be  seen,  so  he  had  a  detail  in  a  boat  to  es- 
tablish flags  on  poles  20  feet  long  at  these  bends.  At  II,  the 
sketcher  draws  pencil  rays  to  such  flags  as  he  can  see  with  the 
expectation  of  intersecting  on  the  same  flags  from  some  subse- 
quently located  station;  he  thus  draws  rays  to  flags  1,  2,  3, 
(on  all  intersecting  rays  the  vertical  angle  should  be  written 
with  pencil  on  the  field  sheet).  To  the  northeast  there  is  a 
ravine  about  a  mile  away  -whose  general  direction  he  can  de- 
termine by  a  single  ray.  There  is  also  a  knoll  about  three- 
quarters  of  a  mile  away  that  he  can  locate  by  intersection,  and 
he  therefore  takes  a  sight  at  its  top  using  a  rock  or  tree  or 
other  easily  recognizable  object,  if  there  is  none  the  center  of 
the  top  of  the  hill  will  do.  This  ray  he  designates  5;  the  vertical 
angle  is  written  along  the  ray.  He  also  takes  rays  to  the 
north  and  south  at  the  foot  of  the  hill,  but  does  not  determine 
the  vertical  angles,  for  he  can  see  that  the  ground  from  the  foot 
of  the  hill  he  is  on  to  the  foot  of  this  knoll  is  practically  level. 

For  the  next  instrument  station  he  selects  the  northern  knoll 
of  this  same  ridge,  and  locates  it  by  resecting  on  AA,  AB,  and 
AC;  the  central  knoll  can  be  located  by  stadia  from  it.  The 
critical  points  «,  b,  c,  d,  e,  are  determined  by  stadia  readings  and 
after  reduction  are  plotted;  a  sighting  ray  is  taken  along  the 
north  foot  of  the  spur  to  the  southeast.  From  this  data  the 
whole  ridge  as  far  north  as  I2a  can  be  plotted.  I2e  is  also 
a  bend  of  the  river.  Rays  are  taken  on  1,  2,  3,  4  and  5,  which 
with  those  from  II  locates  points  1,  2,  3,  and  6.  20  feet  must 
be  subtracted  from  the  elevations  of  the  flags  as  obtained  in 
order  to  get  the  elevation  of  the  river  at  these  bends. 

From  13  the  southern  slope  of  the  low  ridge  is  determined 
and  plotted,  while  rays  to  the  east  and  west  of  the  1090  foot 
hill  determined  from  II  and  12  enable  that  hill  to  be  plotted. 
A  ray  to  4  with  the  one  from  12  determines  the  bend  of  the 
river  at  that  point. 


98  Military  Topography  and  Photography 

Instrument  stations  14,  15,  16,  and  17,  all  determined  by 
resection,  locate  the  critical  points  of  the  river  through  that 
section,  and  the  elevation  of  the  river  at  those  points. 

From  18  the  location  and  elevation  of  that  point  is  deter- 
mined, and  the  bend  of  the  road  near  there.  From  this  point 
all  the  slope  to  the  west  can  now  be  plotted.  From  19  the  road 
crossing  and  the  road  bend  to  the  west  are  determined.  Rays 
from  this  point  along  the  roads  will  determine  the  map  direction 
of  these  roads  from  their  plotted  intersection.  From  17,  the 
direction  of  the  road  to  the  southeast  and  to  the  west  can  be 
determined. 

In  reference  to  that  portion  shown  west  of  the  river,  the  im- 
mediate vicinity  of  AC  would  have  been  plotted  when  that  sta- 
tion was  occupied  for  determination.  From  110  the  slope  to 
the  north  can  be  obtained;  from  111  the  slope  to  the  south  and 
also  the  stream  from  the  northwest;  from  112,  the  slope  from 
that  point  to  the  edge  of  the  swamp  can  be  determined  and 
plotted. 

In  this  illustration  very  few  details  have  been  shown.  This 
in  order  to  make  the  fundamental  principles  of  contouring  the 
clearer.  The  topographer  having  determined  the  location  and 
elevation  of  the  critical  points,  has  the  slope  of  the  ground 
between  right  before  him  from  which  he  should  be  able  to  plot 
his  contour  lines.  In  actual  sketching  when  a  critical  point 
is  determined,  the  lines  may  be  erased,  which  leaves  the  field 
sheet  free  for  other  work  without  a  multitude  of  lines  in  which 
to  lose  determined  data.  In  the  diagram  resection  lines  are 
represented  by  dot-dash  lines ;  intersection  lines  and  stadia 
readings  by  dot  lines.  The  critical  points  of  vegetation  areas, 
location  of  houses,  etc.,  etc.,  are  determined  by  any  of  the 
methods  explained  heretofore :  intermediate  points  are  esti- 
mated. •••*"?. 

Field  notes,  if  kept,  should  be  systematic  and  legible.  The 
following  shows  a  specimen  of  a  field  note  page. 

Title  and  By  Whom  Executed:  Every  completed  field  sheet, 
map  or  sketch  should  have  included  on  it  (1)  the  kind  of  map, 
(2)  by  whom  executed,  (3)  date  of  execution,  (4)  scale,  (5) 


Military  Topography  and  Photography  99 

contour  interval,  (6)   a  simple  graphic  scale  in  yards  or  feet, 
and  (7)  a  simple  slope  scale;    as, 

TOPOGRAPHICAL  SURVEY 

NEAR 

EAGLE  PASS,  TEX  AS. 

BY 

LT  JOHN  DoE.40tklNF 

SEPT.  15tK  1915 
SCALE:  3"  —    1    MILE 


The  title  should  also  include  the  purpose  for  which  the  survey 
is  for  if  this  is  not  apparent  from  the  map  itself;  as,  "Proposed 
Military  Reservation,"  Etc.  The  map  should  also  have  both  a 
true  and  magnetic  arrow.  Should  meridians  of  latitude  be 
plotted  a  true  north  arrow  will  not  be  necessary. 


100  Military  Topography  and  Photography 

FIELD  NOTES,  TOPOGRAPHICAL,  SURVEY 


Fm.       To    Hor.  Dis. 

Azimuth 

Control         Check 

11          AA 

6200 

97°  30'     277°30' 

AB 

11320 

139°33'     319°33' 

AF 

5560 

234°15'       54°15' 

a 

520 

167°56' 

b 

960 

198°06' 

c 

1320 

251°  30' 

d 

560 

227°20' 

e 

1720 

322°12' 

\ 

f 

820 

340°00r 

9 

1080 

13°40' 

h 

760 

80°30' 

1 

2560 

44°35f 

2 

1380 

56°45' 

3 

2160 

121°55' 

5 

3530 

223°20' 

12         AA 

5720 

74°00'     254°00' 

AB 

9080 

Y  4?  <5?°  /)(£>'          Q1  '  ®°  *  f)6)f 
J-O<3    U&          oJLO    \J& 

AC 

6000 

165°58'     345°58' 

a 

730 

205°05' 

b 

600 

257°14' 

c 

1000 

344°12' 

d 

1130 

17°  28' 

e 

360 

-  112°26' 

2 

3160 

8°10' 

1 

4310 

15°00' 

3 

1700 

42°57' 

4 

1900 

148°35' 

5 

3140 

265°10' 

13         AA 

8100 

58°00'     238°00' 

Ver.  Ang.  Elev. 

+           28'  1125 

+           28'  1125 

0  1123 

—  3°51'  1090 
3°17'  1070 
4°25'  1023 

10°45'  1019 

—  3°30'  10W 

45'  1100 

5°33'  1020 

7°58'  10W 

—  2°01'  1015 
3°43'  1015 

—  2°19'  1017 

35'  1090 

+     30'  1125 

+     54'  1124 

+     58'  1125 

7°14'  1032 

—  10°06'  1018 

o  1124 

—.   5°19'  10W 

16°33'  1018 

1°36'  1015 

—  1°12'  1015 
2°54'  1017 

—  2°35'  1019 

38'  1090 

+     45'  1068 


Military  Topography  and  Photography  101 

Sketcher  - Date  

Instr.  Man Inst. 

Recorder - Stadia  Cons 

NEEDLE  REMARKS 

N8£°30'W  Elev. 

N40°30'W  Elev. 

N54°15'E  Elev. 

N12°00'W  Slope  even  to  Col. 

N18°00'E  Back  of  spur  to  N.  E. 

N71Q30'E  Foot  of  spur  to  N.  E. 

N47°15'E  Slope  uniform 

S37°45'E  Slope  uniform 

S20Q00'E  Slope  uniform 

S13°45'W  Slightly  convex  near  sta. 

S80°30'W  Slope  uniform 

S44°30'W  Flag  at  bend  of  river— 

S56°45'W,  Flag  at  bend  of  river— % 

N58°00'W  Flag  at  bend  of  river— - 

N43°15'E  Knoll  to  northeast 

S74°00'W  Elev. 

N47°00'W  Elev. 

N14°00'W  Elev. 

N25°00'E  Slope  cvncave  to  col. 

N77°15'E  Slope  uniform 

S15°45E  To  knoll  with  col.  between 

S17°30'W  Slightly  concave     ' 

N67°30W  Slope  uniform  to  river 

S  8°15'W  See  %  from  II.— 20' 

S15°00'W>  See  1  from  II.— 80' 

S43°00'W  See  3  from  II.— W 

N31°30'W  Flag  at  bend  of  river— 

N85°15'E  See  5  from  II 

n'W  Elev. 


102  Military  Topography  and  Photography 

PHOTO-TOPOGRAPHIC  OPERATIONS 

GENERAL  PRINCIPLES.  In  photo-topographic  surveys,  photo- 
graphs of  the  terrain  are  taken  from  triangulation  stations, 
from  which  the  critical  points  of  the  terrain  are  determined 
by  graphic  methods  and  the  terrain  plotted  in  the  office.  The 
sketcher  does  not  see  the  terrain  and  the  work  is  only  suitable 
where  wide  generalization  is  permissible.  Such  will  usually  be 
the  case  in  the  survey  of  mountains  where  the  difficulty  of  field 
work  ,will  make  photo-topographic  methods  of  double  value. 
Although  the  U.  S.  Government  surveys  have  employed  photo- 
topographic  methods  only  to  a  small  extent,  some  European 
Governments  and  Canada  have  done  so  quite  extensively  and 
with  very  great  success.  In  the  future  topographic  surveys  of 
mountains  2,000  feet  in  height  and  over  will  generally  be  made 
by  photo-topographic  methods. 

PHOTO-TOPOGRAPHIC  INSTRUMENTS.  The  principal  instru- 
ment used  in  photo-topographic  surveys  is  the  camera,  which  is 
of  fixed  focal  length  and  is  mounted  on  a  tripod  much  the 
same  as  a  transit  is.  There  are  a  number  of  different  kinds  of 
survey  cameras  now  manufactured,  some  of  these  are  very  good 
while  others  are  not ;  the  author  would  recommend  the  "photo- 
theodolite"  as  the  most  efficient  and  best  adapted  instrument 
for  the  work  on  the  market.  It  can  be  used  not  only  for  the 
topographic  work  but  also  the  triangulation  work. 

The  photo-theodolite  should  consist  of  a  camera  of  fixed  focal 
length  mounted  on  a  tripod  the  same  as  a  transit.  There 
should  be  leveling  screws  and  level  bubbles ;  a  vertical  axis  and 
horizontal  scale  and  vernier,  so  that  the  camera  can  be  revolved 
in  azimuth  and  set  at  any  angle ;  also  a  horizontal  axis  with 
scale  and  vernier,  so  that  the  camera  can  be  plunged  and  set 
at  any  vertical  angle ;  and  in  addition,  there  may  be  a  telescope 
with  either  horizontal,  or  vertical,  or  better  both,  axis,  with 
the  corresponding  scales  and  verniers.  This  telescope  is.  very 
handy  in  measuring  angles  in  triangulation  work  and  dispenses 
with  the  use  of  a  transit. 

The  camera  should  contain  a  solid  rectangular  reticle  of 
the  same  size  as  the  ground  glass,  which  has  two  cross  wires 
defining  the  vertical  and  horizontal  medians  of  the  same.  These 


FIELD  PHOTO-THEODOLITE 
Courtesy   of   Carl   Zeiss 


104  Military  Topography  and  Photography 

wires  should  be  sufficiently  near  the  ground  glass  to  cast  a  well- 
defined  line  oh  the  negative  during  exposure,  the  photographic 
lines  on  the  negative  furnishing  the  control  for  plotting.  The 
cross  wires  should  be  vertical  and  horizontal,  respectively,  with 
the  level  bubbles,  and  their  intersection  should  be  on  the  optical 
axis  of  the  lens.  When  the  vertical  vernier  reads  "zero"  the 
optical  axis  of  the  lens  should  be  parallel  with  the  plane  of  the 
level  bubbles,  and  perpendicular  to  the  vertical  axis  of  the 
camera.  The  plane  of  the  ground  glass  should  always  be  per- 
pendicular to  the  optical  axis  of  the  lenses. 

Either  plates  or  films  may  be  used.  Films  are  much  more 
easily  transported  in  the  field  and  are  also  more  easily  loaded ; 
either  film  rolls  or  film  packs  may  be  used.  If  plates  are  used, 
the  magazine  plate  holder  will  be  found  much  more  convenient. 

The  Stereo-Comparator:  If  two  views  of  the  same  terrain 
are  taken  in  which  the  camera  in  both  cases  had  the  optical 
axis  of  its  lense  perpendicular  to  the  same  base  line  and  was  a 
short  distance  apart  on  that  base  line,  then  the  distance  to 
any  object  in  the  terrain  can  be  computed  from  the  base  line 
and  the  amouiit  of  parallax  to  that  object  as  shown  from  the 
two  views.  Parallax  as  shown  on  photographic  negatives  is  of 
very  small  magnitude,  and  where  it  is  measured  a  measuring 
instrument  of  great  precision  is  required.  Such  an  instrument, 
known  as  the  Stereo-Comparator,  has  been  invented  by  Dr. 
C.  Pulfrich  of  Jena. 

The  Stereo-Autograph:  Oberleutnant  Ed.  Hitter  von  Orels 
of  the  Militargeogr aphis chen  Institut  in  Wien  has  invented  an 
instrument,  known  as  a  "Stereo-autograph"  which  mechanically 
determines  the  horizontal  and  vertical  position  of  points  from 
a  picture  and  plots  them  on  a  plane  surface  to  any  desired  scale. 

PHOTO-TOPOGRAPHIC  CONTROL.  The  control  for  a  photo- 
topographic  survey  is  an  elaborate  system  of  triangulation :  the 
same  as  that  of  a  regular  topographic  survey.  If  there  is  no 
existing  triangulation  of  the  territory  to  be  surveyed,  it  will  be 
necessary  to  first  measure  a  base  line  and  establish  a  triangula- 
tion system  upon  it  in  the  territory.  If  there  exists  a  triangula- 
tion system,  such  as  the  Geological  or  Coast  &  Geodetic  Survey, 
it  will  have  to  be  reduced  to  a  system  of  smaller  triangles, 


a,  9 


Military  Topography  and  Photography  109 

as  for  sketching,  so  that  the  terrain  can  be  effectively  covered 
with  photographs.  If  a  photo-theodolite  be  used,  the  triangula- 
tion  and  the  photographing  of  the  terrain  can  all  be  done  with 
one  instrument  and  with  one  set-up  at  each  station;  otherwise 
it  will  be  necessary  to  have  a  regular  transit  to  execute  the 
triangulation  work.  Instrument  stations  will  usually  be  pri- 
mary and  secondary  triangulation  stations,  but  tertiary  sta- 
tions established  by  either  photographic  or  transit  resection 
will  also  be  frequently  used.  Where  the  photo-theodolite  is 
used  resection  can  be  made  then  as  with  a  transit  or  plane 
table.  At  each  instrument  station  a  series  of  photographs  are 
taken  covering  the  whole  horizon;  by  photographic  intersec- 
tion, graphically  determined  from  two  or  more  negatives  taken 
from  different  known  stations,  critical  points  are  plotted,  their 
elevations  determined,  and  the  terrain  plotted  therefrom. 

PHOTOGRAPHIC  INTERSECTION.  If  a  negative  be  placed  in  the 
same  position  with  respect  to  the  terrain  as  when  it  was  exposed 
in  the  camera,  the  rays  of  light  connecting  points  on  the  terrain 
and  their  images  on  the  negative  will  all  meet  in  the  same  point. 
This  point  is  just  as  far  back  of  the  negative  as  the  optical 
center  of  the  lens  was  in  front  of  the  dry  plate  during  exposure : 
in  other  words,  this  point  is  just  the  focal  length  of  the  lens 
used,  away  from  the  negative.  From  Fig.  34  it  can  be  easily 
seen  that  when  two  negatives  of  the  same  object  taken  from 
different  positions,  are  plotted  to  point  in  the  same  direction  as 
when  exposed,  the  lines  connecting  the  focal  points,  a  and  b, 
and  the  critical  point  x,  form  a  triangle  which  is  exactly  similar 
to  the  natural  triangle  ABX.  If,  therefore  ab  is  plotted  to 
scale,  then  ax  and  bx  will  also  be  to  scale,  and  the  map  position 
of  X,  can  be  plotted  by  producing  the  photographic  rays  ax 
and  bx  to  the  point  x  where  they  intersect.  See  Fig.  35. 

(1 )  Photographic  Intersection  Without  Orientation:  Hav- 
ing given  two  triangulation  or  instrument  stations,  A  and  B, 
and  an  unknown  critical  point  X,  to  determine  the  map  position 
x  from  two  photographs  of  X — one  taken  from  A  and  including 
in  its  field  B  and  X,  and  the  other  from  B  and  including  in  its 
field  A  and  X,  See  Fig.  35, 


112  Military  Topography  and  Photography 

Procedure:  (1)  From  the  triangulation  data  plot  the 
points  a  and  b  on  the  map  to  scale,  and  draw  a  pencil  ray  ab ; 
(2)  with  dividers  and  reading  scale  measure  the  distance  on 
plate  "A"  from  the  image  ba  to  the  vertical  median  Pp,  and  on 
plate  "B"  from  the  image  ab  to  the  vertical  median  p1;  (3) 

pba 
construct  at  a  an  angle  pab  whose  tangent  equals  — 7-  ,  and  at  b 

an  angle  p*ba  whose  tangent  equals  — 7 — ,  in  which  f  is  the  focal 

length  of  the  lens  of  the  camera  used;  (4)  measure  off  on  the 
lines  ap  and  bp1  the  distance  of  f  in  inches;  (5)  at  p  and  p1 
construct  perpendicular  lines  to  ap  and  bp1,  respectively  (these 
lines  represent  the  horizontal  positions  of  the  negatives  at  the 
time  the  exposures  were  made)  ;  (6)  with  dividers  measure  on 
Negative  "A"  the  distance  from  the  image  point  xa  to  the 
vertical  median  p  and  lay  this  distance  off  on  the  perpendicular 
pba  from  point  p  to  xa,  similarly  measure  the  distance  on  Nega- 
tive "B"  from  the  image  point  Xb  to  the  vertical  medium  p1 
and  lay  this  distance  off  on  the  perpendicular  p*ab  from  point 
p1  to  Xb;  (7)  now  plot  the  lines  axa  and  bx  through  point  a  and 
xa  and  b  and  Xb,  respectively,  producing  them  until  they  inter- 
sect at  a  point  x,  the  map  position  of  the  unknown  point  X. 
Where  the  scale  is  large  and  the  distance  to  the  critical  point  is 
small,  the  plotted  triangle  axb  will  not  be  large  enough  to  con- 
tain a  perpendicular  xaba,  nor  the  actual  length  of  the  negative. 
In  such  cases  it  will  be  better  to  use  a  blank  piece  of  tracing 
paper  plotting  the  line  ab  long  enough  to  contain  the  lengths  aba 
and  bab,  respectively.  This  triangle  can  be  very  easily  reduced 
to  scale  by  drawing  a  line  parallel  to  bx  intersecting  the  side  ab 
at  a  point  from  a  equal  to  the  map  distance  of  AB  then  this 
line  will  intersect  side  ax  at  the  map  position  of  X.  The  dis- 
tances of  f,  bap,  pxa,  etc.,  could  also  be  proportionally  reduced 
by  dividing  the  same  by  some  factor,  as  2,  3,  or  5,  etc. 

(2)  Photographic  Intersection  With  Orientation — Fleem- 
er's  Three-Point  Orientation:  Having  given  two  triangulation 
stations,  A  and  B,  with  a  series  of  photographs  at  each,  to 


Military  Topography  and  Photography  113 

/ 

determine  the  map  position  of  an  unknown  point  X  visible  to 
both  A  and  B. 

In  this  method  the  camera  is  correctly  oriented  at  each  sta- 
tion and  a  series  of  photographs  taken  at  each  to  include  the 
whole  horizon.  Thus  if  the  camera  lens  included  a  field  of 
60°,  it  would  take  six  photographs  at  each  station  and  the 
camera  could  be  set  at  the  following  azimuths:  0°,  60°,  120°, 
180°,  240°,  and  300°.  For  orienting  the  camera  at  the  first 
station,  the  same  methods  as  explained  for  the  plane  table  can 
be  used,  preferably  a  sun  azimuth;  at  the  second  and  succeed- 
ing stations  orientation  shoulcTbe  made  by  back-sighting.  We 
will  assume  that  the  photograph  at  A  which  includes  X  was  set 
at  240°,  and  the  one  at  B  which  includes  X,  at  120°,  as  shown 
in  Fig.  36. 

Procedure:  (1)  Plot  the  points  a  and  b  to  scale  and  at 
each  point  construct  polygons  (hexagons)  in  which  the  per- 
pendicular distance  from  the  center  points  to  the  sides  is  equal 
to  f;  (2)  measure  off  the  distances  xap  and  Xbp1  from  the 
negatives  ("A"240°  and  "B"120°)  and  plot  the  distances  on 
the  proper  side  of  the  proper  polygons;  (3)  now  draw  the 
lines  axa  and  bxb  until  they  intersect  in  a  point  x,  which  is  the 
map  position  of  X. 

PHOTOGRAPHIC  RESECTION.  It  will  occasionally  be  desirable 
to  occupy  an  unknown  point  as  an  instrument  station,  from 
which  to  take  a  series  of  photographs  for  topographic  purposes. 
In  such  cases  where  the  photo-theodolite  is  used,  the  azimuths 
to  visible  triangulation  stations  can  be  read,  recorded,  and  then 
plotted.  The  intersection  of  these  azimuth  rays  is  the  map 
position  of  the  unknown  station.  Where  a  camera  is  used  with 
which  horizontal  angles  cannot  be  read,  the  map  position  of  the 
unknown  station  can  be  determined  if  a  photograph  taken  from 
that  station  contains  at  least  three  visible  plotted  triangulation 
stations. 

(1)  The  Three-Point  Problem:  Having  given  a  photo- 
graphic negative  containing  three  triangulation  stations,  A,  B, 
and  C,  taken  from  an  unknown  station  X,  to  determine  the  map 
position  of  X. 


\-'-;^\ 

X        ///  N 


\       ^ 

-O" 
\ 


\     \ 


Military  Topography  and  Photography 


115 


Procedure:  (1)  Draw  a  line  mn  on  a  piece  of  tracing  paper 
and  upon  it  construct  a  perpendicular  vp  equal  to  the  focal 
length  of  the  camera  lens  (Fig.  37)  ;  (2)  lay  off  on  the  line  mn 
the  distances  va1,  vb1,  and  vc1,  equal,  respectively  to  the  dis- 
tances on  the  negative  MN  of  the  image  points  a,  b,  and  c,  from 
the  vertical  median  vv1 ;  (3)  from  point  p  draw  lines  through 
points,  a1,  b1,  and  c1;  (4)  now  adjust  the  tracing  paper  over 
the  map  so  that  the  lines  pa1,  pb1,  and  pc1  pass  through  the 


FIG.  37 — PHOTO-RESECTION 

plotted  points,  Aa,  Ab,  and  Ac,  and  the  point  p  will  then  be 
over  the  map  position  of  the  unknown  station  X.  Prick  p  so 
as  to  mark  the  position  of  x  on  the  map. 

(2)  The  Five-Point  Problem — Prof.  Schiffner's  Simplified 
Graphic  Construction:  Having  given  a  photographic  negative 
containing  five  triangulation  stations,  A,  B,  C,  D,  and  E,  taken 
from  an  unknown  station  X,  to  determine  -the  map  position 
of  X. 

Procedure:  (1)  from  a  draw  lines  through  the  plotted 
points  b,  c,  d,  and  e  (Fig.  38)  ;  (2)  from  the  negative  mark  the 


116 


Military  Topography  and  Photography 


horizontal  positions  of  the  images  of  A,  B,  C,  D,  and  E  on 
a  straight  edge  of  blank  paper,  labeling  them  ax,  bx,  cx, 
dx,  and  ex;  (3)  apply  this  straight  edge  to  the  blank  paper  so 


PICTURE  TRACE 


FIG.  38 — FivE-PoiKT  PHOTO-RESECTION 

that  points  bx,  dx,  and  ex,  coincide  with  the  lines  ab,  ad,  and  ae, 
respectively,  mark  the  position  of  a  and  draw  then  line  aa^  ; 
(4)  similarly  apply  the  straight'  edge  so  that  the  points  bM 


Military  Topography  and  Photography  117 

Cx,  and  dx,  coincide  with  the  lines  ab,  ac,  and  ad,  respectively, 
mark  a2  and  draw  the  line  aa2 ;  (5)  similarly  apply  the  straight 
edge  so  that  points  ax,  dx,  and  ex,  coincide  with  lines  ba,  bd,  and 
be,  respectively,  mark  b3  and  draw  the  line  bb3 ;  (6)  similarly 
apply  the  straight  edge  so  that  points  ax,  cx,  and  dx,  coincide 
with  lines  ba,  be,  and  bd,  respectively,  mark  b4  and  draw  the  line 
bb4 ;  (7)  lines  aai  and  bb3  will  intersect  at  a  point  R19  and  aa2 
and  bb4  at  R2 — now  through  points  Rx  and  R2  draw  a  straight 
line  intersecting  bd  produced  at  some  point  P,  and  line  ad  at 
some  point  Q;  (8)  from  P  draw  a  line  through  a,  and  from  Q 
a  line  through  b :  their  point  of  intersection  is  the  map  position 
x  of  the  unknown  station  X. 

ORIENTATION  OF  PICTURE  TRACE  BY  THREE-POINT  METHOD. 
Having  given  a  photographic  negative  containing  three  triangu- 
lation  stations,  A,  B,  and  C,  taken  from  a  known  station  S, 
to  orient  the  picture  trace  on  the  map — Prof.  Schiffner's 
Method. 

It  is  usually  better  to  orient  the  picture  trace  by  the  tangent 
method  explained  above,  but  it  may  occasionally  be  desirable  to 
use  the  method  explained  in  this  section. 

Procedure:  (1)  Draw  radials  from  s  through  the  plotted 
points  a,  b,  and  c  (Fig.  39)  ;  (2)  at  a  convenient  point  a^  on 
the  radial  sa,  draw  a  line  parallel  with  radial  sc;  (3)  upon 
this  line  lay  off  distances  aibt  and  biCi  equal  respectively  to 
the  horizontal  distances  of  the  images  of  A  and  B  and  B  and 
C  on  the  negative;  (4)  at  bj  and  Ci  draw  lines  parallel  with 
radial  sa,  producing  them  so  as  to  intersect  radials  sb  and  sc, 
respectively,  and  then  through  these  intersections  draw  a  line 
hh1 — this  line  is  parallel  with  the  desired  position  of  the  picture 
trace ;  (5)  on  line  hh1  lay  off  distances  hb2  and  baC1  equal  respec- 
tively to  aibi  and  biCi,  and  through  points  b2  and  c2,  draw 
parallels  to  radials  sa  intersecting  the  radials  sb  and  sc,  respec- 
tively at  b1  and  c1 ;  (6)  through  b1  and  c1  draw  a  line  inter- 
secting the  radial  sa  at  a1 ;  this  line  a1b1c1  is  the  required  posi- 
tion of  the  picture  trace. 


118 


Military  Topography  and  Photography 


PICTURE  TRACE 


FIG.  39 


DETERMINATION  OF  DISTANCE  WITH  THE  STEREO-COMPARA- 
TOR. Having  given  two  photographs  of  the  same  object,  taken 
with  the  optical  axis  parallel  in  the  two  exposures. 

When  two  photographs  of  the  terrain  are  taken  from  two 
different  stations,  as  Oi  and  O2  in  Fig.  40,  with  the  optical  axes 
parallel,  the  distance  to  any  critical  point  can  be  found  from 

f-b 
the  following  formula :  c=— 5— . 


Military  Topography  and  Photography 


119 


In  Fig.  40,  let  Ot  and  O2  represent  the  instrument  stations ; 
MI  and  M2  the  terrain  photographed  in  each  case;  nil  and  m2 
the  negatives ;  and  B  a  distant  critical  point.  The  optical  axes 
OiMi  and  O2M2  are  parallel  and  the  distance  apart  is  measured. 
All  the  rays  passing  through  a  lens  meet  in  a  point,  from  which 
it  is  evident  that  the  lines  joining  B  to  Oi  and  O2  pass  through 
the  image  points  Bt  and  B2  of  B  on  plates  mi  and  m2.  First  as- 


M 


FIG.  40 


FIG.  41 


sume  that  B  falls  along  the  line  dMj,  then  the  triangle  OiBO2  is 
similar  to  triangle  m2O2B2  and  e  is  to  f  as  b  is  to  d.  This  is 
true  for  all  positions  of  B  within  the  field  of  the  camera  lens. 
Thus  in  Fig.  41,  triangle  OiBO2  is  similar  to  triangle  BO2B2, 
and  6  is  to  di-d2  as  OiB  is  to  B  :O2  ;  Triangle  dBO1  is  similar 
to  triangle  O2m2B,  and  e:f::OiB  :BiO2,  from  which  the  fol- 
lowing proportion  and  equation  are  true:  e:f  ::b  :dt-d2,  or 
_  f-b 
e  —  ~  • 


In  this  equation  f  is  the  focal  length  of  the  camera  lens  and 
b  is  measured,  and  both  are  thus  known;  the  distance  d]-d2  can 


»TZ 


vy 

H 


®l®. 


<s»r  <s> 

r-» 


~  t 


SjfS 

^^:w^L 


at1 


Military  Topography  and  Photography  121 

be  measured  on  the  negative;  and  e  can  be  solved  from  the 
equation. 

To  measure  this  small  distance,  d^da,  accurately,  the  stereo- 
comparator  is  used.  The  principle  of  this  instrument  is  shown 
in  Fig.  42,  in  which  two  vertical  pointers  Si  and  S2  are  brought 
over  the  image  points  nil  and  m2  by  means  of  a  horizontal  mi- 
crometer screw  S.  The  small  distance  di-d2  (or  Xi-x2)  is  read 
from  the  scales.  The  distance  y  of  the  image  point  above  or 
below  the  horizontal  median  HH1?  can  also  be  accurately 
measured  on  the  stereo-comparator. 

DETERMINATION  OF  ELEVATION.  When  the  camera  is  level 
the  horizontal  median  hht  (Fig.  43)  on  the  negative  mark  points 
which  are  of  the  same  elevation  as  the  instrument  station,  and 
since  the  size  of  an  image  varies  inversely  with  the  distance,  the 
difference  in  elevation  between  the  instrument  station  and  any 
photographed  point  can  be  determined  if  the  distance  between 
them  and  the  distance  of  the  photographed  point  above  or  below 
the  horizontal  median  hhi  on  the  negative  are  known.  In  Fig. 
43,  a  represents  one  of  the  foci  of  the  camera  lens,  xa  the 
image  point,  x  the  critical  point,  hh1  the  horizontal  median  of 
the  negative,  and  o  a  point  directly  below  x  and  of  the  same 
elevation  as  a.  xah  and  xo  are  parallel  to  each  other  and  the 
triangle  axih  and  axo  are  proportional.  Therefore,  axa:ax:: 
Xah  :xo,  or  f  :1 :  :d  :e,  in  which  f  is  the  focal  length  of  the  camera 
lens,  1  the  distance  from  the  instrument  station  to  the  critical 
point,  d  is  the  distance  above  or  below  the  horizontal  median, 
and  e  the  difference  in  elevation  between  the  instrument  station 

I'd 
and  the  critical  point.     Then  e  =  ±  — 7-- 

e  and  1  are  given  in  feet,  while  d  and  f  are  given  in  inches, 
d  is  plus  or  minus  according  as  to  whether  it  is  above  or  below 
the  horizontal  median.  Where  the  critical  point  is  several 
miles  distance,  a  correction  should  be  made  for  the  earth's 

1-d         7.92L2 

curvature,  which  makes  the  equation :    e  =  rb  — j-  —  — r^ 

I  l^a 

where  L  is  the  distance  in  miles. 


Military  Topography  and  Photography  123 

When  the  camera  is  plunged,  or  inclined  to  a  plus  or 
minus  vertical  angle,  the  following  equation  must  be  used: 

1-d  7.92-L2 

e=  -+-  7 5-7  -  -  -i-   1  tan  0,  where  0  is  the  vertical 

"   f  cos   0  12 

angle  of  the  optical  axis  (camera).  Where  E  is  the  elevation 
of  the  instrument  station  and  E1  that  of  the  critical  point,  then 
the  complete  equation  is  :  E1  =  E  ±  e. 

When  the  camera  is  plunged  the  horizontal  axis  must  be 
level  so  that  no  matter  what  the  vertical  angle  at  which  the 
camera  is  set  the  optical  axis  is  in  the  same  vertical  plane.  In 
photographic  intersection  and  resection  work  it  makes  no  dif- 
ference whether  the  camera's  optical  axis  is  horizontal  or 
plunged  providing  the  camera's  horizontal  axis  is  level. 


CHAPTER  III 

MILITARY  SKETCHING 

By  "military  sketching"  rapid  field  topographical  sketching 
is  usually  meant — where  simple  instruments  are  used  and  the 
control  work  is  rather  rudimentary  as  distinguished  from  topo- 
graphical surveying  where  instruments  of  precision  are  used 
and  the  control  work  is  very  accurate.  It  is  so  considered  and 
used  in  this  book. 

MEASURING  AND  PLOTTING  DIRECTION 

(1)  WITH  PRISMATIC  COMPASS.  When  the  prismatic  com- 
pass is  used  for  determining  direction,  the  field  sheet  is  usually 


*  PRISMATIC  COMPASS  AND  CLINOMETER 

a  transparent  tracing  paper  placed  over  a  paper  protractor. 
This  paper  protractor  should  be  rectangular  in  shape  and 
about  the  same  size  as  the  sketching  board.  The  circular  scale, 
or  protractor  scale,  should  be  8,  10,  or  12  inches  in  diameter 
•and  graduated  into  degrees.  The  prismatic  compass  is  held 
level  in  the  hand  at  the  height  of  the  eye  and  the  vertical  wire 
of  the  sighting  vane  is  brought  in  line  with  the  object  whose 

*  Courtesy  of  W.  &  L.  E.  Gurley. 


Military  Topography  and  Photography  125 

direction  is  to  be  determined.  While  looking  through  the  eye 
piece  the  scale  of  the  prismatic  compass  is  also  visible  from 
which  the  magnetic  azimuth  to  the  unknown  point  is  read  direct- 
ly. This  azimuth,  it  should  be  remembered,  has  the  north  in- 
stead of  the  south,  as  the  point  of  reference,  and  should  the 
zero  on  the  protractor  be  south  a  correction  of  180°  must  be 
made  to  it.  Having  determined  the  azimuth  with  the  prismatic 
compass,  the  direction  (line)  is  plotted  with  triangles  as  ex- 
plained in  Chapter  II,  page  85. 

When  opaque  paper  is  used  for  the  field  sheet,  a  T-square 
and  semiprotractor  are  used  J. or  the  plotting.  The  azimuths 
are  read  with  the  prismatic  compass  after  which  they  are 
plotted  as  explained  in  Chapter  II,  page  84. 


*SKETCHING  BOARD  AND  TRIPOD 

(2)  WITH  SKETCHING  BOARD  ORIENTED.  This  method  is 
used  where  the  sketching  board  contains  a  compass,  usually  a 
trough  compass,  by  means  of  which  it  can  be  oriented.  After 
the  board  has  been  oriented  a  pin  is  stuck  in  the  plotted 
instrument  station,  and  around  this  pin  a  ruler  is  pivoted  to 
such  a  position  that  the  edge  of  the  ruler  is  brought  into  line 
with  the  unknown  critical  point.  A  pencil  ray  now  drawn  along 
the  ruler's  edge  will  mark  the  direction  line  to  the  unknown 
critical  point.  This  is  a  very  simple  method,  dispenses  with  the 
carrying  of  a  detached  compass,  and  is  the  method  generally 
used  in  the  field  for  rapid  sketching. 

*  Courtesy  of  W.  ft  L.  E.  Gurley. 


126 


Military  Topography  and  Photography 


MEASURING  AND  PLOTTING  DISTANCE 

(1)  BY  PACING  OR  TIMING.  If  the  sketcher  knows  the 
length  of  his  regular  pace,  by  pacing  the  distance  between  two 
points  their  distance  apart  can  be  easily  computed  or  plotted 
from  the  number  of  paces  required  to  be  taken  to  walk  from 
one  to  the  other.  The  distance  so  paced  is  quite  accurately 
determined  providing  the  ground  is  fairly  open,  level  and  solid. 
If  it  be  up  or  down  grade  a  reduction  must  be  made,  and  also, 
if  the  ground  be  muddy,  heavy,  or  otherwise  have  a  retarding 
effect  on  one's  gait. 

Reduction  for  Grades: 

5°  10°  15°  20° 

Up  Grades:  10%  20%  25%  30% 

Down  Grades:         4%  8%  10%  12% 

The  distance  between  two  points  can  also  be  equally  accu- 
rately computed  from  the  time  necessary  to  walk  from  one  point 


*TALLY 

to  the  other,  allowing  for  the  same  factors  as  in  pacing.     Tim- 
ing is  much  more  accurate  than  pacing  where  mistakes  in  count- 

*  Courtesy  of  W.  &  L.  E.  Gurley. 


Military  Topography  and  Photography 


127 


ing  are  very  easily  made,  although  the  pace  tally  and  pedometer 
will  eliminate  this  objection.  Distance  is  also  determined  by 
observing  the  time  it  takes  a  horse  to  travel  the  distance  at 
established  gaits.  Bicycles  and  carriage  wheels  can  also  be 
used,  the  number  of  revolutions  of  a  wheel  times  its  circumfer- 
ence giving  the  distance.  The  revolutions  are  recorded  on  an 
odometer,  but  if  the  sketcher  has  no  odometer  a  white  rag  tied 


*ODOMETER 

on  a  spoke  or  a  felloe  will  enable  the  revolutions  of  the  wheel 
to  be  easily  counted. 

(2)  BY  INTERSECTION.    Intersection  gives  directly  the  map 
distance  to  any  intersected  point,  from  which  the  ground  dis- 
tance can  be  found  by  applying  a  reading  scale  to  it. 

(3)  BY  RESECTION.     Resection  also  gives  directly  the  map 
distance  to  any  resected  point,  from  which  the  ground  distance 
can  be  found  by  applying  a  reading  scale  to  it. 

(4)  BY  ESTIMATION.     If  the  sketcher  has  had  much  ex- 
perience in  topographical  surveying  and  has  cultivated  an  "eye" 
for  ground  distances,  he  should  be  able  to  estimate  distances 
about  as  accurately  as  they  can  be  paced.     No  inexperienced 
sketcher,  if  it  can  be  avoided,  should  attempt  to  estimate  dis- 
tances, for  such  estimating  is  nothing  more  than  mere  guessing. 
The  beginner  should  always  measure  distances  at  first  by  stadia. 

"Courtesy  of  W.  &  L.  E.  Gurley. 


128  Military  Topography  and  Photography 

intersection,  and  other  accurate  means  in  order  to  cultivate  a 
good  "eye"  for  distances.  Topographical  surveying  is  an  ex- 
cellent preliminary  training  for  rapid  military  sketching,  and 
all  officers  that  can  should  do  some  topographical  surveying. 

The  following  facts  in  regard  to  the  appearance  of  objects 
under  different  conditions,  given  in  the  Small  Arms  Firing 
Manual  (1913),  should  be  tested  and  verified: 

Objects  seem  nearer — 

(a)  When  the  object  is  in  a  bright  light. 

(b)  When  the  color  of  the  object  contrasts  sharply  with 

the  color  of  the  background. 

( c)  When  looking  over  water,  snow,  or  a  uniform  surface 

like  a  wheat  field. 

(d)  When  looking  from  a  height  downward. 

(e)  In  the  clear  atmosphere  of  high  altitudes. 

Objects  seem  more  distant — 

( a)  When  looking  over  a  depression  in  the  ground. 

( b)  When  there  is  a  poor  light  or  a  fog. 

(c)  When  only  a  small  part  of  the  object  can  be  seen. 

(d)  When  looking  from  low  ground  upward  toward  higher 

ground. 

CONSTRUCTION  OF  WORKING  SCALE.  For  plotting  distances 
a  working  scale  should  be  constructed  with  units  corresponding 
to  the  units  in  which  the  ground  distances  are  to  be  measured. 
The  unit  used  may  be  the  pace,  stride,  wheel-revolutions,  time, 
etc.  ~  'w  ( 

To  Construct  a  Pace- Working  Scale 

Measure  off  with  a  steel  tape  or  chain  a  distance  of  800  to 
1000  yards.  Pace  this  distance  forward  and  back  at  least  six 
times.  It  is  better  to  pace  this  distance  on  different  days,  than 
to  do  the  pacing  all  on  one  day.  Record  the  number  of  paces 
each  time,  discarding  any  obvious  mistakes  in  counting,  divide 
the  sum  of  these  paces  by  the  number  of  times  paced,  the  quo- 
tient is  the  average  number  of  paces  for  the  distance ;  divide  the 
distance  by  the  average  number  of  paces  and  this  quotient  will 
give  the  average  length  of  pace.  For  example : 


Military  Topography  and  Photography  129 

Distance  =  1000  yards. 

Date :  No.  of  Paces : 

April  5  1146 

"  5  1149 

"  6  1145 

"  6  1147 

"  7  1146 

"  7  1149 

6882  ~  6  =  1147  =  Average  number  of 
paces. 

1000  yards  -r-  1147  —  .875-yards  =  31*5  inches  =  Length 
of  pace. 

It  is  desired  to  construct  a  scale  about  three  inches  long,  to 
be  used  in  road  sketching.  The  scale  of  road  sketches  is  three 
inches  to  one  mile.  Divide  the  denominator  of  the  R.F.  of  the 
road  sketch  by  the  length  of  the  pace  in  inches.  The  R.F.  of 
three  inches  to  one  mile  is  1/21,120.  Therefore: 

21,120  -r-  31.5  =  670.5  =  number  of  paces  to  one  inch  on 
map. 

A  scale  about  three  inches  long  would  contain  (670.5  X  3) 
2011.5  paces.  We  shall  therefore  select  2000  paces  as  the 
length  of  our  working  scale,  as  this  number  is  also  one  that 
can  be  easily  subdivided. 

2000  ~  670.5  =  2.98  inches  =  Length  of  .scale. 

Measure  off  2.98  inches  and  divide  the  length  into  five  equal 
parts,  as  explained  in  the  case  of  the  construction  of  a  reading 
scale,  p.  21.  Equal  parts  will  then  represent  400  paces,  a  very 
convenient  number  for  subdividing.  Construct  a  scale  as  shown 
in  the  diagram.  Fig.  44. 

Construct  working  scales  from  the  following  data: 

(1)  Working  Scale  of  Strides: 

Scale  of  map  =  Six  inches  to  one  mile. 
Distance  paced  =  880  yards. 
Average  number  of  strides  =  500. 

(2)  Time  Working  Scale : 

Scale  of  sketch  =  Three  inches  to  one  mile. 
Distance  paced  =  880  yards. 
Average  time  =10  minutes. 


JtO     j 
}fC\lt>W 

Mil 


/re 

\*\$a 


FIG.  44 


Military  Topography  and  Photography 

(3)  Working  Scale  of  Wheel-revolutions: 

Scale  of  sketch  =  Three  inches  to  one  mile. 
Distance  traveled  =  One  mile. 
Number  of  revolutions  =  674. 

(4)  Working  Scale  for  Horse  Walking: 

Scale  of  sketch  =  Three  inches  to  one  mile. 

Distance  traveled  =  One  mile. 

Average  time  =  15  minutes. 

When  sketching  with  a  board  that  is  oriented  at  each  set-up, 
it  is  more  convenient  to  have  the  working  scale  right  on  the 
alidade  ruler ;  but  where  a  prismatic  compass  is  Used,  it  is  more 
convenient  to  have  the  working  scale  on  a  cardboard  attached 
to  the  sketching  board,  plotting  distances  therefrom  by  means 
of  a  pair  of  dividers. 

MEASURING  AND  PLOTTING  SLOPES 

VERTICAL  ANGLE  MEASUREMENT.  (1)  With  Slope  Bodrd: 
For  diagram  of  a  board  see  Fig.  45.  The  slope  (sketching) 


FIG.  45 

board  is  held  in  a  vertical  plane  and  at  such  an  angle  that  by 
sighting  along  the  top  edge  of  the  board,  the  object  whose  verti- 
cal angle  is  desired  is  just  seen.  When  that  position  is  attained 
the  plumb-bob  string  should  be  held  firmly  in  position  by 
pressing  the  finger  against  it.  The  intersection  of  the  string 
with  the  circular  scale  is  the  vertical  angle  required. 


132 


Military  Topography  and  Photography 


In  a  slope  board  it  is  obvious  that  a  line  passing  through  the 
center  and  the  zero  of  the  circular  scale  should  be  exactly 
perpendicular  to  the  top  edge  of  the  board. 

(2)  With  the  Clinomete^ :  The  service  clinometer  is  held 
in  such  a  vertical  position  that  the  object  is  sighted  on  the 
cross  wire.  The  vertical  angle  is  read  at  the  same  time  the 
sight  is  taken.  The  service  clinometer  is  based  on  a  very  sound 
principle — that  of  gravity.  The  gravity  scale  oscillates  very 
much,  yet  with  a  fairly  steady  hand  and  good  eye,  vertical 
angles  within  one-fifth  of  a  degree  should  be  read. 

In  the  Abney  Clinometer  the  telescope  is  sighted  on  the  ob- 
ject and  held  in  that  position  while  the  scale  is  brought  to  a 


*ABNEY  CLINOMETER 

level  position  by  means  of  an  attached  spirit  level,  visible  while 
sighting  through  the  telescope.  In  that  position  the  scale  is 
clamped  and  read. 

ELEVATION  WITH  ANEROID  BAROMETER.  The  use  of  the 
Aneroid  barometer  in  determining  differences  in  elevations  has 
already  been  discussed  (Chapter  I,  page  6).  Its  use  is  quite 
extensive  in  rapid  military  sketching. 

STADIA  REDUCTION.  When  the  distance  and  vertical  angle 
are  known,  the  difference  in  elevation  in  feet  can  be  obtained  by 
multiplying  the  tangent  of  the  vertical  angle  by  the  ground 
distance.  To  solve  such  problems,  a  stadia  table  or  stadin 
computer  can  be  used.  The  use  of  Cox's  Stadia  Computer  has 
already  been  explained  (Chapter  I,  page  81). 

*  Courtesy  of  W.  &  L.  E.  Gurley. 


Military  Topography  and  Photography 


133 


*SURVEYING  ANEROID  BAROMETER 

USE  OF  SLOPE  SCALES.  Where  two  points  are  plotted  and 
the  vertical  angle  between  them  is  known  the  difference  in  eleva- 
tion between  them  can  be  found  by  applying  directly  to  them  a 
slope  scale  for  the  known  angle  and  counting  the  number  of  con- 
tour intervals  included  by  the  two  points. 

PLOTTING  SLOPES.  The  same  general  principles  governing 
the  plotting  of  slopes,  as  explained  in  topographical  survey  are 
here  applicable.  Not  so  high  a  degree  of  precision,  however,  is 
expected. 

PLOTTING  CHARACTER  OF  TERRAIN 

While  the  conformation  of  the  terrain  is  being  plotted  by 
means  of  contour  lines,  the  features  of  the  terrain  are  at  the 
same  time  represented  by  means  of  conventional  signs  which  are 
usually  suggestive  of  the  thing  represented. 

STREAMS,  LAKES,  ETC.  In  sketching,  the  courses  of  streams, 
and  the  shores  of  lakes  will  usually  be  plotted  by  determining 
a  sufficient  number  of  points  along  them  to  control  their  plot- 

*  Courtesy  of  W.  &  L.  E.  Gurley. 


134 


Military  Topography  and  Photography 


FIG.  46 

ting  within  the  degree  of  accuracy  required.  These  points  are 
determined  by  either  resection,  intersection,  or  other  methods  of 
determining  the  location  of  critical  points.  Where  a  stream  or 
shore  line  is  hid  in  underbrush,  various  means  will  have  to  be  de- 
vised for  their  survey  as  explained  in  the  chapter  on  Topo- 
graphical Surveying. 

Contour  lines  follow  the  banks  of  streams  up-stream  before 
crossing  until  they  intersect  the  water  surface  of  the  stream 
where  they  cross  the  river  in  a  straight  line.  They  do  not  as  is 
frequently  supposed,  follow  the  conformation  of  the  bottom  of 
the  stream.  The  reason  for  this  is  quite  evident:  contour  lines 
along  the  banks  would  coincide  for  long  distances  and  make 
plotting  and  map  reading  unnecessarily  difficult ;  on  the  other 
hand,  to  make  the  necessary  soundings  of  rivers  to  determine 
their  depth  would  be  so  laborious  arid  expensive  as  to  justify  the 
same  only  in  Engineering  Surveys.  In  military  surveys,  streams 


Military  Topography  and  Photography  135 

are  classified  as  fordable  and  unfordable.     Only  the  depths  of 
fords  along  unfordable  streams  are  determined. 

VEGETATION.     See  Chapter  II,  page  93. 

LINES  OF  COMMUNICATIONS.     See  Chapter  II,  page  93. 

TOWNS  AND  BUILDINGS.     See  Chapter  II,  page  94. 

CLASSES  OF  SKETCHES 

- 

Military  sketches  are  divided  into  two  general  classes — road 
sketches  and  area  sketches.  Area  sketches  are  further  divided 
into — position  sketches,  outpost  sketches,  and  place  sketches. 
All  these  have  already  been  described. 

KINDS  OF  SKETCHING 

Sketching  may  be  done  either  by  working  alone  or  in  con- 
junction with  others.  Where  sketching  is  done  by  several  work- 
ing in  conjunction,  they  all,  of  course,  must  use  the  same  pri- 
mary control;  otherwise  it  will  be  almost  impossible  tp  join 
their  sketches  to  form  a  single  map  of  the  area. 

INDIVIDUAL  SKETCHING.  (1)  Individual  Road  Sketching: 
Road  sketching  is- usually  executed  by  making  a  traverse  of  the 
road.  This  traverse  is  the  control  work  of  the  sketch.  Dis- 
tances along  the  road  can  be  measured  by  pacing,  timing,  num- 
ber of  wheel  revolutions,  etc.  By  making  proper  allowances  for 
climatic  conditions,  grades,  etc.,  a  surprising  degree  of  accuracy 
can  be  attained. 

The  sketching  board  at  each  instrument  station  can  be  ori- 
ented either  by  compass  or  by  back  sight.  The  latter  method 
is  more  accurate,  but  takes  much  more  time.  Every  change  of 
direction  and  of  slope  is  a  critical  point.  The  sketching  board 
may  be  set  up  at  every  change  of  direction  in  the  road  or  at 
every  other  change.  In  the  latter  method  only  the  compass 
method  of  orientation  can  be  used.  The  procedure  in  both 
methods  will  be  described. 

Back-sight  Method:  First,  set  up  the  sketching  board  at  A 
(first  instrument  station)  ;  orient  by  compass  or  other  method; 
select  an  arbitrary  point  a  to  represent  A  on  the  sketch ;  stick 
a  pin  at  a  and  pivot  the  alidade  ruler  so  as  to  sight  B ;  draw  an 
indefinite  pencil  ray  ab' ;  pace  the  distance  AB  and  mark  it  off 
on  the  line  ab'  by  means  of  the  working  scale:  this  point  on  ab' 
is  the  map  position  of  B. 


t 


I 

1 

1 

1 

I 

1 

I 

t'" 
§ 

PIG.  47 — BACK-SIGHT  TRAVERSE 


Military  Topography  and  Photography  137 

SECOND  :  Set  up  the  sketching  board  at  B ;  stick  a  pin  at  b 
and  place  alidade  ruler  along  line  ab;  turn  sketching  board, 
without  moving  alidade  ruler,  until  A  is  sighted;  the  sketching 
board  is  then  oriented. 

Third:  With  sketching  board  clamped,  pivot  alidade  ruler 
around  pin  stuck  at  b  until  C  is  sighted;  draw  an  indefinite 
pencil  ray  be';  draw  a  parallel  line  along  ab  to  represent  the 
road.  Repeat  these  operations  at  each  change  of  direction. 

Compass  Method:  First:  Set  up  at  A;  orient  sketching 
board  by  compass ;  select  an  arbitrary  point  a  to  represent  A 
on  the  sketch ;  stick  a  pin  at  a,  and  pivot  working  ruler  on  it 
until  B  is  sighted ;  draw  an  indefinite  ray  ab'  along  the  edge  of 
the  ruler. 

Second:  Pace  the  distance  to  B,  and  mark  it  off  along  the 
line  ab' ;  then  pace  the  distance  from  B  to  C,  making  a  note  of  it. 

Third :  Set  up  at  C  and  orient  sketching  board  by  compass ; 
stick  a  pin  at  b  and  pivot  the  working  ruler  on  it  until  B  is 
sighted ;  draw  an  indefinite  line  be' ;  now  mark  the  distance  BC 
off  along  be':  this  line  be  will  represent  the  section  BC  of  the 
road. 

The  compass  method  can  also  be  used  in  setting  up  at  every 
station.  Cross  roads,  road  junctions,  etc.,  are  also  critical 
.points,  and  the  direction  of  each  intersecting  road  should  also 
be  plotted. 

The  terrain  is  sketched  *4  mile,  or  440  yards,  to  either  side ; 
at  each  station  the  vertical  angles  to  lateral  critical  points,  and 
of  points  within  the  distance  of  the  next  change  of  direction 
along  the  road  are  read  by  means  of  the  clinometer,  or  slope 
board;  the  distance  to  such  critical  points  are  estimated,  from 
which  the  contours  are  plotted  by  means  of  a  slope  scale,  or  the 
difference  in  elevation  is  found  by  means  of  a  stadia  computer 
or  table  and  the  contours  so  spaced  as  to  show  the  character  of 
the  slope.  It  is  quite  evident  that  the  slope  scale  method  is  the 
much  more  rapid  one.  The  vertical  angle  to  the  next  change  of 
direction  in  the  road  is  read,  the  distance  between  paced,  or 
timed,  which  gives  all  the  data  necessary  to  determine  the  eleva- 
tion of  the  next  instrument  station.  The  elevations  of  instru- 
ment stations  are  thus  carried  forward. 


/K 


fc'. 


FIG.  48 — COMPASS  TRAVERSE 


FIG.  49 — ROAD  SKETCHING 


FIG.  50 — POSITION  SKETCHING 


Military  Topography  and  Photography  141 

Individual  Position  Sketching:  In  any  kind  of  sketch- 
ing the  sketcher  must  use  some  kind  of  a  control,  or  his  work 
will  be  inconsistent.  While  this  fact  is  quite  evident  to  the  skill- 
ful sketcher,  the  beginner  does  not  think  of  a  base  of  control ;  he 
usually  tries  to  reproduce  each  feature  of  the  terrain  indepen- 
dently and  without  reference  to  other  features ;  but  the  impos- 
sibility of  this  is  soon  demonstrated  to  him  by  a  warped  sketch 
and  a  confused  idea  as  to  the  real  requirements  for  successful 
sketching.  ^ 

When  given  a  position  to  sketch,  the  sketcher  should  select 
two  prominent  points,  visible«^nd  accessible  to  each  other;  the 
distance  between  them  may  be  determined  by  pacing,  timing, 
estimation,  or  any  other  method  of  distance  determination  used 
in  rapid  sketching.  Even  where  this  distance  is  estimated,  the 
sketch  will  be  consistent  within  itself,  and  all  points  determined 
from  them  will  be  in  proper  relation  with  one  another:  should 
the  distance  be  determined  more  accurately  later  on,  the  scale 
can  be  changed  to  show  it.  (Fig.  50.) 

These  two  points  will  determine  a  base  line.  The  sketching 
board  is  set  up  at  each  of  these  points  in  term,  orienting  the 
board  in  each  case— at  the  first  station  by  compass,  at  the 
second  by  back-sight  on  the  first;  radiating  lines  are  drawn 
towards  all  critical  points  within  the  area,  the  vertical  angles 
determined  and  recorded,  at  each  of  the  two  stations.  The 
radiating  lines- drawn  from  these  two  stations  towards  the  same 
critical  points  will  intersect  and  determine  the  map  position  of 
such  critical  points.  The  vertical  angle  from  both  stations  to 
the  same  critical  point,  and  the  elevation  of  that  point  as  deter- 
mined from  the  two  stations  should  check  within  a*  few  feet.  The 
elevation  of  one  end  of  the  base  line  will  usually  be  assumed  and 
the  elevation  of  the  other  end  determined  from  it. 

Where  the  area  is  large,  auxiliary  base  lines  may  be  estab- 
lished by  intersection  from  the  primary  one,  so  that  the  whole 
area  will  be  covered  by  prominent  critical  points  determined  by 
triangulation.  The  location  of  minor  critical  points  and  points 
between  are  determined  by  resections  and  estimations  based  on 
the  adjacent  intersected  points.  The  whole  area  should  be 
covered  by  the  sketcher  to  detect  all  features  that  might  be 
omitted  or  unseen  by  occupying  only  the  high  points. 


FIG.  51 — OUTPOST  SKETCHING 


Military  Topography  and  Photography  143 

It  should  be  remembered  that  rapid  position  sketches  are  not 
extended  topographical  surveys,  and  hence  the  degree  of  control 
is  not  nearly  so  great. 

(3)  Individual  Outpost  Sketching:     The  control  work  for 
an  outpost  sketch  will  generally  be  a  Base  Line  along  the  line  of 
observation.     From  the  extremities  of  this  Base  Line,  intersec- 
tions can  generally  be  taken  on  a  sufficient  number  of  critical 
points  in  the  foreground  to  control  the  sketching.     If  it  is  im- 
practicable to  measure  the  distance  between  two  suitable  points 
on  the  line  of  observation  directly,  as  where  some  impassible  or 
difficult  ground  lies  between  them,  their  distance  apart  may  be 
determined  in  the  same  manner  as  for  passing  impassible  objects 
in  plane  surveying,  or  their  distance  apart  may  be  determined 
from  an  auxiliary  base  line  in  rear,  say  on  the  line  of  resistance. 
The  diagram  in  Fig.  51,  in  which  C  and  D  are  two  prominent 
points  on  the  line  of  observation,  and  A  and  B,  two  points  on  a 
line  to  the  rear,  illustrates  this  method. 

The  sketching  of  the  terrain  up  to  the  line  of  observation  and 
a  short  distance  beyond  will  usually  be  comparatively  easy,  but 
the  sketching  of  the  terrain  to  the  front  towards  the  enemy  will 
depend  upon  tactical  conditions :  otherwise  the  sketching  of 
outpost  positions  present  no  special  problems. 

(4)  Place  Sketching:  Place  sketching,  or  Eye  Sketching, 
as  it  is  sometimes  called,  is,  as  its  name  indicates,  the  sketching 
of  a  portion  of  the  visible  terrain  from  a  single  point  of  observa- 
tion.    This  problem  will  often  be  met  with  in  scouting,  as  where 
reconnoitering    the   enemy's    outpost   position,    entrenchments, 
camp,  etc.     Where  such  a  sketch  can  be  supplemented  with  a 
photograph  of  the  terrain,  it  should  be  done.    In  sketching  from 
a  single  point  of  observation,  no  base  of  control  can  be  secured. 
The  sketching  board,  or  pad,  should  be  oriented  as  correctly  as 
possible ;   an  arbitrary  point  selected  on  the  paper  to  represent 
the  point  of  observation;  from  this  point,  using  and  sighting 
with  a  working  ruler  if  possible,  radiating  lines  should  be  drawn 
towards  prominent  or   critical  points  in  the  foreground;  the 
distance  to  these  points  are  estimated  and  measured  off  on  the 
sketch  with  a  working  ruler;  the  vertical  angle  to  these  points 
should  also  be  measured,  or  estimated,  and  recorded. 


Military  Topography  and  Photography 

With  the  location  and  elevation  of  these  selected  points,  thus 
determined,  used  for  control,  an  experienced  sketcher  will  be  able 
to  produce  a  fairly  accurate  representation  of  the  ground. 
Such  sketches  will  usually  have  to  be  executed  with  rapidity,  and 
all  elaborate  methods  are  eliminated. 

(5)  Memory  Sketching:  Memory  sketching  is  the  repre- 
sentation, in  the  form  of  a  sketch,  produced  entirely  from  the 
memory.  They  are  usually  of  trails  and  roads.  For  control 
the  sketcher  should  estimate  and  plot  first  the  most  important 
road  or  trail,  and  then  estimate  and  plot  all  other  roads,  trails, 
and  other  objects  with  respect  to  and  based  upon  it.  . 

In  such  sketches,  the  conformation  of  the  ground  is  rarely 
shown  and  then  only  in^a-  very  general  way.  Contour  lines  will 
seldom,  if  ever,  be  used.  The  relief  method  can  be  most  suc- 
cessively employed.  The  general  conformation  can  be  indicated 
by  writing  "hills,"  "planes,"  etc.,  at  their  estimated  locations. 
The  character  of  the  terrain  should  be  shown  as  remembered. 

Any  person  can  make  a  memory  sketch  of  the  terrain  with 
which  they  are  familiar.  The  author  has  often  had  uneducated 
Filipinos  to  sketch  the  roads  and  trails  of  the  terrain  for  con- 
siderable distances  on  the  ground,  by  means  of  which  he  was 
able  to  traverse  unknown  ground  with  ease.  Memory 
sketches  of  roads  and  trails,  on  paper  or  sketched  on  the  ground 
is  the  common  method  of  showing  the  way  to  strangers,  both  in 
civilized  and  semi-civilized  countries.  Such  sketches  present  no 
difficulty  in  execution ;  they  are  of  course  only  approximately 
to  scale,  and  points  may  be  considerably  out  of  proper  relation 
with  one  another,  but  this  is  realized  and  allowed  for  by  the 
reader. 

COMBINED  SKETCHING.  Where  the  sketching  of  an  area  is 
executed  by  several  sketchers  working  in  conjunction,  the  area 
should  be  divided  into  sections  and  a  section  assigned  to  each 
sketcher  or  sub-party.  A  base  line  or  traverse  will  be  used  for 
the  control  of  each  section,  but  in  order  that  the  sections  may  be 
compiled  and  a  consistent  map  of  the  whole  produced,  the  rela- 
tion of  the  control,  both  horizontal  and  vertical,  of  the  several 
sections  to  each  other  must  be  known.  This  may  be  obtained  by 
using  a  base  line  or  traverse  common  to  all,  or  the  sections  using 


Military  Topography  and  Photography  145 

for  their  control  parts  of  a  single  base  line  running  clear  across 
the  whole  area. 

(1 )  Combined  Road  Sketching:  Where  the  roads  of  an  area 
are  to  be  sketched,  each  road  running  towards  the  front  should 
have  a  sub-party  assigned  to  it.  Each  sub-party,  except  the 
left  (or  right)  one  should  have  at  least  one  chief  and  one  assis- 
tant— the  left  (or  right)  sub-party  will  usually  need  only 
one  sketcher.  Over  the  whole  there  should  be  a  chief  sketcher 
and  such  assistants  as  necessary.  Where  the  parallel  roads  to 
the  front  are  far  apart,  or  the  cross  roads  numerous,  each  sub- 
party  should  be  increased  with  the  necessary  assistants.  Com- 
bined road  sketching  will  usually  be  done  by  mounted  sketchers. 

If  possible  the  sketching  should  start  from  a  road  traversing 
the  entire  front,  in  which  case  a  base  line  or  traverse  can  be 
made  of  this  road,  and  each  sub-party  use  that  portion  running 
through  its  section  as  its  primary  control.  Beginning  at  the 
left  end,  the  whole  party  will  follow  the  chief,  sketching  this 
traverse  road.  At  each  road  running  to  the  front  this  sketcher 
will  give  the  necessary  data  (usually  a  carbon  copy  of  the 
sketching  throughout  the  section)  to  the  sub-party  assigned  to 
that  road,  in  order  that  he  may  proceed  at  once  to  its  sketching. 
The  carbon  copy  will  show  the  traverse  road  throughout  the 
section,  the  initial  portion  of  the  road  to  the  front,  the  con- 
tours, and  in  addition,  the  elevation  in  feet  of  the  road's  junc- 
ture with  the  road  to  the  front. 

Should  a  lack  of  time  render  this  procedure  impossible,  each 
sub-party  will  go  to  its  initial  point  at  once  and  begin  sketching. 
In  such  cases,  a  mounted  sketcher  should  ride  rapidly  from  one 
initial  point  to  another,  and  give  each  sub-party  the  elevation 
of  its  initial  point.  For  this  purpose  the  aneroid  barometer  is 
used.  Each  sub-party  must  plot  the  line  of  direction  of  the 
cross  road  at  the  first  set-up,  so  that  its  road  sketch  can  be 
applied  to  the  primary  control  traverse  (cross  road)  which  will 
be  made  simultaneously  with  the  sketching  to  the  front. 

Each  sub-party  traverses  the  road  assigned  to  it.  The 
sketching  of  the  road  is  usually  done  by  the  chief  of  the  sub- 
party.  Whenever  a  cross  road  is  met,  one  of  the  assistants  of 
the  sub-party  is  assigned  by  its  chief  to  traverse  and  sketch  it 


$*.!,- 


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FIG.  52 — COMBINED  ROAD  SKETCHING 


Military  Topography  and  Photography  147 

from  the  road  junction  to  within  1/4  mile  of  the  road  belonging 
to  the  sub-party  to  the  left  of  it :  the  chief  will  sketch  the  cross 
roads  for  1/4  mile  to  the  right  of  the  main  road.  At  a  distance 
of  %  mile  from  the  main  road  each  cross  road  to  the  right 
should  be  marked  with  a  conspicuously  posted  piece  of  paper 
or  cardboard  containing  the  number  of  the  sub-party.  This 
poster  will  show  the  sketcher  of  the  party  to  the  right  just 
where  to  stop,  otherwise  he  would  sketch  clear  to  the  main  road 
running  to  the  front. 

It  is  very  desirable  that  each  sub-party  on  the  left  should  be 
even  with  or  slightly  in  advance  of  the  sub-party  on  its  right, 
and  to  make  this  probable  the  sub-parties  should  be  started  in 
succession  from  left  to  right.  Should  the  right  sub-party  have 
started  off  first,  the  procedure  would  be  reversed:  each  sub- 
party  sketching  just  %  mile  to  its  left  and  to  within  %  mile  of 
the  road  to  its  right.  The  party  last  starting  sketches  to  within 
14  mile  on  both  sides  of  its  road. 

The  conformation  of  the  ground,  character  of  the  vegetation, 
etc.,  will  be  filled  in  to  within  %  mile  on  both  sides  of  all  roads : 
the  intervening  ground  is  not  sketched. 

'(%)  Combined  Position  Sketching:  The  area  to  be  sketched 
will  be  divided  into  sections  and  a  sub-party  consisting  of  a  chief 
and  one  assistant  assigned  to  each ;  over  the  whole  there  will  be 
a  chief  sketcher  and  such  assistants  as  necessary.  The  width  of 
each  section  should  not  as  a  rule  exceed  %  mile.  It  may  extend 
any  distance  towards  the  front ;  for  a  day's  work  it  should  not 
extend  over  two  miles.  For  control,  a  traverse  along  the  near 
side  of  the  area  may  be  run,  or  the  near  side  of  each  section  de- 
termined by  triangulation  from  a  conveniently  located  base  line. 
The  principle  of  the  latter  is  shown  in  the  diagram  in  Fig.  55 
for  outpost  sketching. 

If  there  is  not  sufficient  time  to  make  either  of  the  above  con- 
trols, the  widths  of  the  sections  are  determined  by  estimation 
and  the  elevation  of  the  initial  point  in  each  section  is  deter- 
mined by  occupying  these  points  in  rapid  succession  with  an 
aneroid  barometer.  As  soon  as  each  sub-party  receives  the  ele- 
vation of  its  initial  station  it  begins  sketching.  The  procedure 
will  be  about  as  follows:  Initial  points  (a,  b,  c,  d?  and  e,  Fig. 


150  Military  Topography  and  Photography 

53),  marking  the  points  of  divisions  between  sections  are  estab- 
lished on  the  near  side  or  base  line,  which  points  should  be 
marked  with  streamers  or  stakes.  The  chief  of  sub-party  No.  1 
will  set  up  at  a;  orient;  mark  off  a  line  aa'  at  90°  to  the  base 
line  ae;  he  will  then  sketch  a  strip  200  or  300  yds.  wide  to  the 
limiting  point  a',  and  then  along  the  front  a'b'.  The  chief  of 
sub-part}7  No.  2  and  the  assistant  of  sub-party  No.  1,  the  for- 
mer doing  the  sketching,  will  set  up  at  b ;  orient ;  strike  off  a 
line  bb'  at  90°  to  BE.  They  will  then  sketch  a  strip  200  or 
300  yds.  wide  along  the  line  bb'  to  b'.  At  b',  the  chief  of  sub- 
party  No.  2,  will  cut  his  field  sheet  in  two  along  the  line  bb',  and 
give  its  left  half  to  the  assistant  of  sub-party  No.  1,  who  will 
sketch  along  the  line  b'a'  until  he  meets  his  own  chief.  At  this 
point  the  chief  will  take  the  sketch  his  assistant  has  and  fasten 
it  on  his  own  field  sheet  in  its  proper  map  position.  If  the 
length  of  the  base  line  ab  has  been  measured,  this  can  be  easily 
done;  if  not,  the  sketches  along  a'b'  will  be  placed  in  contact 
with  the  lines  aa'  and  bb'  parallel  to  each  other.  The  sub-party 
having  its  border  sketched,  in  returning  to  the  base  line,  sketches 
the  area  within  the  sketched  borders.  The  chief  of  sub-party 
No.  2  sketches  to  the  right  along  b'c'  to  the  .point  where  he 
meets  his  own  assistant,  and  so  on.  When  the  sketch  is  com- 
pleted, each  section  is  trimmed  to  its  limiting  lines  and  the 
several  sections  pasted  together  to  represent  the  whole  area. 
This  method  will  insure  contour  lines  matching  when  the 
sketches  are  brought  together,  and  make  adjustments  unneces- 
sary. See  Fig.  54. 

(3)  Combined  Outpost  Sketching:  The  organization,  con- 
trol, and  procedure  in  outpost  sketching  is  much  the  same  as 
that  in  position  sketching,  except  the  ground  to  the  front  will 
not  be  traversed  in  sketching,  the  same  being  done  from  the  line 
of  observation.  The  sketchers  will  be  divided  into  sub-parties 
of  one  or  more  sketchers  each,  all  under  a  chief.  Each  party 
will  be  assigned  a  section,  designated  not  only  by  limiting  points 
along  the  line  of  observations,  but  also  by  limiting  points  to  the 
front.  The  outpost  position  will  generally  be  in  a  convex  line, 
so  that  the  sections  will  be  much  wider  to  the  front  than  at  the 
line  of  observation. 


152  Military  Topography  amd  Photography 

For  control,  a  traverse  may  be  made  of  the  line  of  observa- 
tion, or  the  limiting  points  of  sections  along  the  line  of  observa- 
tion determined  by  triangulation  from  a  primary  base  line  to  the 
rear.  The  diagram  in  Fig.  55  will  illustrate  the  general  prin- 
ciple of  the  latter  method  of  control. 

Using  the  limiting  points  of  their  sections  on  the  line  of  ob- 
servation, the  sub-parties  will  all  begin  at  the  same  side  of  their 
sections,  which  should  be  designated  by  the  chief.  The  sketch- 
ing boards  are  set  up  on  these  initial  points,  oriented,  and  rays 
taken  from  this  point  to  all  the  prominent  points  in  their  re- 
spective sections.  The  sketchers  then  go  to  their  other  base 
points  and  from  there  take  rays  on  these  same  critical  points  of 
their  sections,  the  intersection  of  these  rays  determining  the 
map  position  of  these  critical  points.  The  vertical  angles  to 
these  points  should  be  taken  from  both  base  points. 

The  horizontal  and  vertical  locations  of  the  critical  points  of 
a  section  are  thus  determined  from  which  the  conformation  of 
the  ground  can  be  sketched.  If  the  sub-pa'rties  begin  at  their 
right  base  point,  each  sketcher  should  begin  to  sketch  from  the 
left,  and  when  he  has  completed  a  strip  200  or  300  yards  wide, 
he  should  make  a  tracing  or  carbon  copy  of  it  and  send  it  to  the 
sub-party  on  his  left.  This  tracing  or  copy  should  have  the 
elevation  of  contours  marked  on  it.  This  will  enable  each  sub- 
party  to  the  left  to  connect  his  work  on  that  of  the  sub-party  to 
the  right. 

RAPID  SKETCHING 

GENERAL  PRINCIPLES  :  There  is  no  art  to  which  the  state- 
ment: "Practice  makes  perfect,"  is  more  applicable  than  to 
sketching.  Only  by  much  practice  and  experience  can  any  de- 
gree of  perfection  be  attained.  Some .  are  naturally  better 
adapted  to  sketching — learn  it  more  quickly — and  do  it  more 
accurately  and  neatly.  This  does  not,  however,  prevent  anyone 
from  learning  the  art.  The  average  non-commissioned  officer 
and  scout  will  learn  sketching  more  in  the  manner  of  trade : 
without  thoroughly  understanding  the  theory  and  principles  in- 
volved ;  sketching  operations-  can  be  executed  by  a  mere  knowl- 
edge of  how  to  do  them,  and  a  fairly  accurate  map  produced. 
Before  any  high  standard  of  sketching  can  be  attained,  the 


Military  Topography  and  Photography  153 

sketcher  must  acquire  a  keen  sense  of  ground  distance,  direction, 
and  degree  of  slopes,  and  also  a  correct  appreciation  of  map 
distances.  The  beginner  should  therefore  make  haste  by  work- 
ing slowly  at  first,  until  he  has  acquired  an  "eye"  for  map 
elements.  Even  secondary  critical  points  should  at  first  be  deter- 
mined by  measuring  them:  estimates  in  the  beginning  will  be 


*ARMY  SKETCHING  CASE 

nothing  more  than  guesses,  and  all  "guesses"  should  be  elimi- 
nated in  sketching.  With  practice  the  number  of  secondary 
critical  points  whose  determination  may  be  made  by  estimation 
will  gradually  increase  until  it  will  be  necessary  to  measure 
only  the  controlling  critical  points. 

KNOWLEDGE  OF  MAP  DISTANCES.     The  sketcher  should  ac- 
quire an  accurate  conception  of  small  lineal  units.   He  should  be 

*Courtesy  of  W.  &  L.  E.  Gurley. 


154<  Military  Topography  and  Photography 

able  to  estimate  an  inch  within  1/20  of  an  inch — to  divide  an 
inch  into  2,  3,  4,  5,  8,  and  10  equal  parts  by  estimation.  Given 
a  map  of  a  given  scale,  he  should  be  able  after  a  short  inspection 
of  it,  to  estimate  the  distances  of  100  yards,  500  yards,  1000 
yards,  and  other  multiples  of  100  yards.  Given  a  map  of  given 
scale  and  contour  interval,  he  should  after  short  inspection,  tell 
the  degree  of  all  slopes  by  observing  the  spacing  of  the  contours. 
To  him  a  topographical  map  should  stand  out  as  a  real  model 
would.  These  are  prerequisites  to  rapid  sketching. 

KNOWLEDGE  OF  GROUND  DISTANCE.  The  sketcher  must  have 
an  accurate  conception  of  100  yards  and  multiples  of  it  to  1000 
yards — of  1200  yards,  1500  yards,  and  2000  yards.  The  effect 
of  light,  slopes,  and  intervening  objects  in  making  objects  ap- 
pear to  be  nearer,  or  farther  away  must  be  known. 

KNOWLEDGE  or  DIRECTION.  The  determination  of  direction 
is  perhaps  the  easiest  map  element  to  determine,  yet  from  that 
mere  fact  it  is  sometimes  the  most  fruitful  source  of  error.  The 
determination  of  the  cardinal  points  of  the  compass  has  already 
been  considered.  The  sketcher  should  have  an  accurate  concep- 
tion of  90°,  and  be  able  to  divide  a  right  angle  (90°)  into  two 
equal  parts  of  45°  each,  also,  into  three  equal  parts  of  30°  each, 
and  also  into  iline  equal  parts  of  10°  each.  The  directions  from 
one  point  to  other  points  on  the  terrain  should  be  plotted  bv 
sighting  along  a  working  ruler  to  such  points,  keeping  the  ruler 
in  contact  with  a  pin  stuck  at  the  map  position  of  station  from 
which  the  directions  are  being  taken. 

KNOWLEDGE  OF  SLOPES.  A  sketcher  should  be  able  to  esti- 
mate the  degree  of  any  slope  with  accuracy,  and  he  must  know 
just  how  far  apart  to  space  the  contours  to  represent  the  slope 
without  using  a  slope  scale  or  counting  the  number  of  contour 
intervals.  This  requires  much  practice  in  measuring  slopes  and 
in  the  use  of  slope  scales  in  plotting  them.  He  must  also  acquire 
the  ability  of  estimating  elevations  directly  and  know  the  effect 
of  intervening  slopes  upon  the  appearance  of  elevations.  The 
beginner  will  usually  guess  the  degree  of  a  slope,  either  from  the 
top  or  bottom,  at  about  twice  or  more  its  actual  amount. 

ACCURACY  AND  NEATNESS.  '  The  learner  of  any  trade  or  pro- 
fession must  have  good  tools  to  begin  with,  and  it  is  especially 


Military  Topography  and  Photography  155 

necessary  for  skctchers  to  have  good  sketching  instruments  in 
the  beginning.  They  cause  him  to  do  his  work  more  carefully 
and  accurately,  and  enable  him  to  acquire  a  truer  appreciation 
of  map  and  ground  elements.  The  commander  or  instructor 
who  furnishes  his  men  with  sawed-off  fence  boards,  soft-lead 
pencils,  etc.,  for  sketching,  in  order  to  simulate  war  conditions, 
will  not  produce  skctchers  whose  work  can  be  depended  upon; 
but  after  a  man  has  once  acquired  the  art  of  sketching  by  learn- 
ing with  good  instruments,  he  will  be  able  to  produce  a  good  map 
or  sketch  .whatever  the  materials  at  hand. 


CHAPTER  IV 

PHOTOGRAPHY* 

THE  CAMERA.  Cameras  may  be  divided  into :  first,  those 
using  plates  and  those  using  films;  and  second,  those  in  which 
the  focus  is  fixed  and  those  in  which  the  focus  is  not  fixed.  Both 
fixed-focus  and  focusing  cameras  can  be  used  either  for  plates 
or  for  films,  or  for  both  according  to  their  construction. 

THE  Box.  In  fixed-focus  cameras,  the  box  is  light  tight  and 
covered  with  leather :  at  the  front  of  the  camera  there  are  the 
lens  and  shutter;  at  the  back,  some  arrangement  for  loading 
the  camera  either  with  plates  or  films ;  on  top  there-  is  a  view 


CAMERA — FIXED  Focus 

finder.  The  box  of  the  fixed-focus  camera  has  no  ground  glass 
and  it  is  unnecessary  to  estimate  the  distance  of  the  object,  the 
camera  being  in  focus  for  all  distances.  Such  cameras  are 
necessarily  much  more  bulky  than  the  focusing  camera  which 
telescopes  to  a  small  size  when  closed. 

In  the  focusing  cameras,  the  front  side  drops  down  to  form 
the  base  for  the  focusing  of  the  camera.  On  this  side,  or  base, 
is  a  track  upon  which  the  lens  frame  may  be  run  back  and  forth 
to  any  position  desired.  The  lens  frame  is  attached  to  the  box 
of  the  camera  by  means  of  a  light-tight  telescopic  bellows.  At 
the  back  there  is  some  arrangement  by  means  of  which  the 
camera  may  be  loaded  either  with  plates  or  films,  or  both.  Some 
focusing  cameras  have  a  ground  glass  at  the  back  upon  which 

*For    a    complete    treatise    on    amateur    photography,    the    reader    is    referred    to 
How  to  Make  Good  Pictures,  Published  by  the  Eastman  Kodak  Co.,  Rochester,  N,  Y. 
fpourtesy  of  Eastman  Kodak  Co, 


Military  Topography  and  Photography 


157 


the  image  of  an  object  can  be  focused;  others  do  not,  and  with 
them  it  is  necessary  to  estimate  the  distance  to  the  object.     The 


TREMO  CAMERA — FOCUSING 


latter  will  always  have  a  view  finder  attached  to  the  lens  frame. 
For  distances  of  100  feet  or  more,  the  focus  of  the  camera  is 


*3A.    AUTOGRAPHIC  KODAK 

practically  the  same,  so  that  for  such  distances  the  lens  may  be 
set  at  the  100  foot  mark  on  the  scale.     For  less  than  100  feet, 

*Courtesy  of  Eastman  Kodak  Co. 


158  Military  Topography  and  Photography 

the  distance  must  be  accurately  estimated,  and  the  nearer  the 
object  to  the  camera,  the  more  accurate  the  estimate  must  be. 
The  scale  on  the  base  board  of  a  focusing  camera  should  oc- 
casionally be  tested  to  see  if  it  is  in  adjustment.  To  do 
this  it  is  only  necessary  to  bring  in  focus  on  the  ground  glass 
the  image  of  an  object  that  is  slightly  more  than  100  feet  from 
the  camera.  The  pointer  should  then  be  directly  opposite  the 
100-foot  division  on  the  scale.  If  it  is  not,  the  position  of  the 
scale  should  be  so  changed  that  the  pointer  is  opposite  the 
100-foot  division.  Should  the  camera  contain  no  ground  glass, 
the  back  may  be  removed  and  a  detached  ground  glass  held  in 
the  position  the  plate  or  film  would  occupy,  and  the  image  of  a 
distant  object  focused  upon  it. 


*3A.     GRAFLEX 

THE  LENS.  With  the  modification  that  the  camera  box, 
must,  of  course,  be  light  tight,  of  good  construction,  and  the 
shutter  a  good  one,  the  statement  that,  "The  lens  is  the 
camera,"  is  true.  Standard  lenses,  however,  are  made  of  dif- 
ferent capacities,  and  any  such  lens  will  do  just  as  good  work 
as  any  other  such  lens  within  the  limits  of  its  capacity. 

*Courtcsy  of  Eastman   Kodak   Co. 


Military  Topography  and  Photography  159 

OPTICS  OF  LENSPJS 

FOCAL  LENGTH.  The  focal  length  of  a  lens  is  the  distance 
between  the  optical  center  of  a  lens  and  the  vertical  plane  of 
the  image  which  it  forms  of  a  distant  object.  This  vertical 
plane  of  the  image  will  be  the  surface  of  the  film  or  plate  in, 
or  the  ground  glass  of,  the  camera  when  it  is  focused  on  a  dis- 
tant object.  In  cases  of  simple  lenses,  this  distance  can  be 
measured  directly  from  the  mechanical  center  of  the  lens:  but 
in  case  of  most  hand  cameras  in  which  double  combination  lenses 
are  used,  of  which  the  optical  center  is  not  the  mechanical  cen- 
ter, the  equivalent  focus  must  be  determined,  by  which  is  meant, 
the  distance  also  between  the  optical  center  of  the  double  com- 
bination and  the  ground  glass  when  the  camera  is  focused  on  a 
distant  object. 

To  measure  the  focal  length  of  a  simple  lens:  The  diagram 
in  Fig.  56  explains  itself — C  is  a  candle;  L,  the  lens;  and  S, 


FIG.  56 

a  white  cardboard,  or  screen.  'L  is  placed  in  the  center  of  a 
yard  stick,  while  C  and  S  are  so  adjusted  with  respect  to  each 
other  that  the  image  of  the  candle  flame  on  the  screen  is  exactly 
the  same  size  as  the  flame  itself.  In  such  a  position,  the  candle 
and  screen  should  be  exactly  the  same  distance  from  the  lens 
and  one-half  of  either  distance  is  the  focal  length  of  the  lens. 
If  it  is  desired  to  check  this  result,  the  following  method  is  used : 
The  candle  and  lens  are  placed  at  a  convenient  distance  apart 
on  the  yard  stick,  and  the  screen  is  brought  to  such  a  position 
that  the  image  is  most  distinct.  Then  the  focal  length  of  the 
lens  is  found  from  the  following  formula:  F  =  CL  X  LS  -f- 
(CL  +  LS).  This  operation  should  be  carried  out  for  several 
positions  of  the  lens,  and  the  mean  value  of  the  F's  obtained, 
taken  as  the  most  probable  value  of  F. 


160  Military  Topography  and  Photography 

To  determine  the  equivalent  focus  of  a  double  combination 
lens:  The  camera  is  focused  on  a  distant  object  and  the  posi- 
tion of  the  lens  on  the  base  board  (position  of  scale  pointer)  is 
marked  on  the  base.  A  ruler  with  well-defined  division  marks  is 
then  tacked  on  the  wall  at  the  height  of  the  camera  when  placed 
on  a  tripod  or  table,  and  the  camera  is  brought  to  such  a  dis- 
tance from  the  ruler  on  the  wall,  that  the  image  of  the  ruler  on 
the  ground  glass,  when  the  camera  is  focused,  is  exactly  the  same 
size  as  the  ruler  itself.  To  determine  this,  a  pair  of  dividers 
may  be  set  for  exactly  one  inch  on  the  ruler,  and  the  dividers  so 
set,  applied  to  the  image  on  the  ground  glass.  When  the  camera 
is  so  adjusted  that  the  image  in  focus  is  exactly  the  same  size 
as  the  ruler,  as  indicated  by  the  dividers  the  position  of  the 
pointer  is  again  marked  on  the  base.  The  distance  between 
these  two  marks  is  the  equivalent  focus  of  the  double  combina- 
tion lens  tested. 

SPEED  OF  LENSES.  There  is  a  common  but  fallacious  idea, 
that  the  speed  of  a  lens  depends  upon  the  glass  of  the  lens.  It 
is  true  that  glass  of  different  densities  have  different  indices  of 
refraction,  and  light  travels  through  them  with  different  rates  of 
speed.  But  those  differences  are  so  small,  the  lens  being  so 
thin,  that  variations  in  speed  due  to  the  material  of  the  lens 
must  be  entirely  omitted.  It  must  be  admitted  that  speed  as 
used  in  the  statement,  "speed  of  lenses,"  is  not  correctly  used 
according  to  the  definition  of  that  word.  By  speed  of  lenses  is 
meant  the  "amount"  of  light  which  enters  a  lens  within  a  given 
time :  this  is  independent  of  the  rate  at  which  light  travels,  since 
that  rate  is  constant ;  it  is  dependent  only  upon  the  size  of  the 
lens — the  size  of  the  opening  through  which  the  light  passes. 
Since  the  areas  of  two  circles  are  proportional  to  the  square  of 
their  diameters,  a  lens  which  has  a  diameter  twice  as  great  as 
another  lens  will  have  a  speed  (capacity)  four  times  greater 
than  the  other  lens. 

The  value  of  lenses  of  large  diameter,  or  light  capacity,  or 
speed  as  it  is  commonly  called,  is  quite  evident  in  photographing 
moving  objects.  In  such  cases  an  image  is  desired  only  at  one 
position  during  the  motion,  which  requires  an  exposure  of  very 
short  duration;  as,  1/100,  1/500,  or  1/1000  of  a  second,  or  an 


Military  Topography  and  Photography  161 

instantaneous  exposure  as  all  exposures  of  over  1/25  of  a  second 
are  called.  If  such  an  exposure  is  to  be  transmitted  to  the 
sensitive  plate,  sufficient  light  must  have  entered  through  the 
lens  during  that  time  to  effect  the  proper  chemical  change  in 
the  sensitive  solution  of  the  dry  plate  emulsion.  Here  a  large 
lens  would  permit  sufficient  light  to  enter  while  a  smaller  one 
would  not. 

The  speed  of  a  lens  is  always  listed  or  expressed  in  the  ratio 
of  its  focal  length  to  its  focal  length  divided  by  the  diameter  of 
its  largest  stop  (largest  diameter  at  which  it  may  be  used). 
Thus,  if  the  focal  length  of  a  lens  were  4.75"  and  the  diameter 
of  its  largest  stop,  .75",  its  <speed  is  f.6.3.  If  the  stop  of  this 
lens  is  reduced  so  that  the  ratio  is  f.8,  then  the  speed  of  this  lens 
at  that  stop  is  no  greater  than  the  speed  of  another  lens  of  the 
same  focal  length  whose  largest  opening  is  only  f.8.  The  sizes 
of  shutter  stops  are  generally  given  in  the  following  ratios — 
f.4.5;  f.6.3;  f.8,  f.ll;  f.16;  f.22;  f.32;  f.64;  and  f.128.  The 
diameter  of  these  stops  vary  directly  with  their  ratios.  The 
speed  of  f.64  is,  therefore,  four  times  as  great  as  that  of  f.128. 

DEPTH  OF  Focus.  By  the  depth  of  focus  is  meant  the  range 
within  the  maximum  and  minimum  limits  with  respect  to  dis- 
tance in  which  all  objects  in  the  field  will  be  in  focus  on  the 
ground  glass  at  the  same  time.  This  depth  increases  with  the 
distance  from  the  lens;  it  varies  inversely  with  the  size  of  the 
stop;  and  it  varies  inversely  with  the  focal  length  of  the  lens. 
A  fast  lens  closed  down  to  f.128  will  have  the  greatest  depth 
of  focus.  An  exposure  at  this  stop  will  take  a  proportionally 
longer  time  to  let  in  sufficient  light  to  produce  the  proper 
chemical  effect  on  the  sensitive  plate. 

A  great  depth  of  focus  is  desired  in  photographing  the  ter- 
rain. f.16  or  f.32,  will  usually  be  found  sufficient  in  such  work. 
Fixed  support  must  of  course  be  used  in  time  exposures. 

DEFINITION.  The  definition  of  a  lens  is  the  sharpness,  and 
clearness  of  the  image  which  it  produces  of  an  object.  This  de- 
pends entirely  upon  the  quality,  purity,  and  perfection  in  cur- 
vature of  the  lens.  A  lens  will  therefore  give  good  definition 
only  within  the  limits  in  which  it  is  ground  in  perfect  curvature. 
Should  there  be  imperfection  in  curvature,  in  the  lens  there 
will  be  different  images  of  the  same  object  which  will  produce 


162 


Military  Topography  and  Photography 


a  haze  instead  of  a  well-defined  image.  The  diagram  in  Fig.  57 
will  illustrate  this.  A  perfect  lens  produces  only  a  single  image 
of  the  same  object. 


FIG.  57 — LACK  OF  DEFINITION 

ASTIGMATISM.  By  astigmatism  is  meant  a  defect  in  a  lens 
in  which  the  arcs  of  curvature  through  all  meridians  arc  not  of 
the  same  degree.  Such  lenses  do  not  render  horizontal  and 
vertical  lines  equally  sharp.  Non-astigmat  lenses  are  free  from 
astigmatism. 

STYLE  OF  LENSES 

SINGLE  LENSES.  Single  lenses  are  made  in  two  forms,  menis- 
cus and  plano-convex.  The  meniscus  form,  giving  the  greater 


*Fio.    58 — PLANO-CONVEX    LENS  *Fio.    59 — MENICUS    LENS 

definition,  is  always  employed  in  the  best  cameras.  The  single 
lenses,  are  made  of  two  different  kinds  of  glass — crown  and  flint, 
cemented  together,  called  combinations,  in  order  to  make  them 
achromatic. 

NONACHROMATIC  LENSES.  When  light  passes  through  a 
glass  prism,  the  light  will  be  dispersed  into  its  primary  colors 
upon  leaving  the  prism.  Light  passing  through  a  simple  lens, 


FIG.  60 — SINGLE  PRISM  (Non- Achromatic) 
^Courtesy  of  Eastman  Kodak  Co. 


Military  Topography  and  Photography 


163 


made  of  one  kind  of  glass,  will  do  the  same.     Such  lenses  are 
called  nonachromatic,  and  are  not  used  in  standard  cameras. 

ACHROMATIC  SINGLE,  LENSES.  If,  however,  another  prism  of 
the  same  kind,  or  another  prism  with  a  different  index  of  refrac- 
tion of  proper  shape  be  used  in  conjunction  with  the  first  prism, 
the  primary  colors  will  be  reassembled  into  one  white  ray  upon 
leaving  the  second  prism.  This  principle  is  followed  in  the  con- 


A      B 

NON- ACHROMATIC. 

FIG.  61 


struction  of  achromatic  single  lenses,  so  called,  out  of  crown  and 
flint  glass,  cemented  together. 

The  effect  of  nonachromatic  lenses  is  to  produce  a  number  of 
different  sized  images  of  the  same  object,  one  for  each  primary 
light  color.  Only  one  of  these  images  can  be  in  the  focus  at  the 
same  time  and  the  combined  image  will  have  neither  definition 
nor  clearness,  being  a  blur  of  several  images.  This  is  illustrated 


FIG.  62 — DOUBLE  PRISM  (ACHROMATIC) 

in  the  diagram  in  Fig.  61,  where  C  is  the  visual  focus,  while  A 
and  B  are  images  of  different  colored  light  rays. 

RAPID  RECTILINEAR  LENSES.  Rapid  rectilinear  lenses  are 
double  combination  lenses  of  two  single  achromatic  meniscus 
lenses.  Each  combination  is  composed  of  two  kinds  of  glass, 
crown  and  flint,  cemented  together  with  a  transparent  cement, 
called  balsam.  The  combinations  are  denoted  as  front  and  roar 
combinations,  respectively,  and  are  mounted  in  the  ends  of  a 
brass  tube,  which  in  the  common  hand  camera  forms  part  of  the 

*Courtesy  of  Eastman  Kodak  Co. 


164 


Military  Topography  and  Photography 


shutter;  the  leaves  of  the  shutter  being  between  the  two  com- 
binations. The  focal  length  of  either  the  front  or  rear  combi- 
nation is  much  greater  than  that  of  both  combinations  together 
and  from  this  fact,  the  rear  combination  may  be  used  alone 


*Fic.  63 — ACHROMATIC  LENS 

with  success  in  telo-photographic  work,  as  where  a  picture 
of  a  single  distant  object  is  desired. 

This  double  combination  is  called  rapid  rectilinear  from 
the  fact  that  it  renders  the  straight  lines  of  a  picture  without 
distortion.  It  is  also  called  symmetrical  or  convertible,  accord- 
ing as  to  whether  the  focal  lengths  of  the  combinations  are  equal 
or  not.  With  the  convertible  rapid  rectilinear  lens,  the 
photographer  has  in  fact  three  distinct  -lenses  of  different  focal 
lengths — the  double  combination,  the  rear  combination  alone, 
and  the  front  combination  alone. 

The  rapid  rectilinear  lens  is  the  kind  with  which  most  hand 
cameras  are  equipped,  and  it  will  be  found  sufficient  for  most 
work.  It  is  next  to  the  anastigmat  lens  in  quality. 


*Fio.  64 — DOUBLE  COMBINATION  LENS 

ANASTIGMAT  LENSES.  Anastigmat  lenses  are  the  best  lenses 
made.  They  have  the  highest  speed,  the  best  definition,  and 
are  entirely  free  from  astigmatism.  The  anastigmat  lenses  are 

*Courtesy  of  Eastman  Kodak  Co. 


Military  Topography  and  Photography  165 

much  more  highly  corrected,  the  surface  being  ground  in  per- 
fect curvature;  they  are  calculated  up  on  formulae  that  permit 
them  to  be  worked  at  much  greater  openings,  and  they  thus 
permit  more  light  to  enter  within  a  given  time  and  are  there- 
fore much  more  rapid.  The  definition  even  at  the  larger  stops 
is  as  great  as  that  for  inferior  lenses  at  smaller  stops.  The 


Object 


Ima<3e 


FIG.  64 

superiority  of  this  lens  in  taking  moving  objects  is  at  once 
apparent.  Used  with  a  focal  plane  shutter,  exposure  of  1/1000 
of  a  second,  or  even  faster,  may  be  made. 

THE  SHUTTER 

GENERAL  PRINCIPLES.  The  shutter  is  the  mechanism  by 
which  the  duration  of  the  admittance  of  light  into  the  camera 
is  controlled.  The  requisites  of  a  good  shutter  are — 1st,  the 
mechanism  should  work  with  precision,  remaining  open  during 
the  period  for  which  it  is  set ;  2nd,  the  diaphram  leaves  should 
be  light  tight  when  the  shutter  is  closed;  and  3rd,  all  parts  of 
the  dry  plate  should  be  exposed  to  the  light  an  equal  time. 
The  focal  plane  shutter  is  the  only  one  that  absolutely  fulfills 
the  last  conditions.  The  present  automatic  shutter,  however, 
reduces  the  inequalities  of  time  to  such  an  extent  as  to  eliminate 
all  noticeable  effects  in  the  picture. 

In  addition  to  the  shutter  diaphram,  there  is  an  adjustable 
stop,  by  means  of  which  the  size  of  the  light  aperture  is  con- 
trolled. At  the  bottom  of  the  front  of  the  shutter  is  a  lever 
containing  a  pointer- — by  moving  this  pointer  to  the  right  or 
left,  the  size  of  the  stop  is  changed.  The  stop  scale  contains 
the  numbers,  f.6.3.  .  .  .  f.128,  and  by  setting  the  pointer 
opposite  the  proper  number,  the  exact  size  of  stop  desired  is 
secured. 


166  Military  Topography  and  Photography 

The  speed  of  shutters  ranges  from  "time"  exposure  down  to 
1/100,  and  even  faster  than  1/1000  of  a  second,  according  to 
the  kind  of  shutter.  When  the  time  pointer  is  set  on  "T" 
(Time),  one  pressure  of  the  air  bulb  opens  the  shutter,  while 
a  second  pressure  is  required  to  close  it:  the  shutter,  therefore, 
can  be  kept  opened  any  length  of  time  on  "T."  If  the  indica- 
tor is  set  on  "B"  (Bulb),  the  shutter  is  opened  on  the  pressure 
of  the  bulb  and  closes  when  that  pressure  is  released.  The 
other  speeds  on  the  common  automatic  shutter  are  generally 
1  second,  1/5,  1/25,  1/50  and  1/100  of  a  second.  These  speeds 
work  automatically,  the  shutter  opening  at  the  pressure  on  the 
bulb  and  closing  at  the  indicated  time  without  the  control  of 
the  release.  Focal  plane  shutters  work  as  fast  as  1/1500  of  a 
second.  Exposures  faster  than  1/25  of  a  second  are  called 
instantaneous  exposures. 

STYLES  OF  SHUTTERS 

There  are  a  large  number  of  different  kinds  of  shutters,  but 
all  may  be  grouped  into  three  general  classes — simple,  auto- 
matic, and  focal  plane  shutters.  The  simple  shutter  is  used 
mostly  in  box,  or  fixed  focus  cameras.  It  consists  of  disk  wheel 
which  has  a  series  of  different  sized  holes,  or  stops,  with  their 
centers  on  the  same  inner  circumference  of  that  disk.  The 
center  of  this  disk  is  so  attached  to  the  camera  front  that  when 
the  disk  is  revolved,  the  center  o'f  each  hole,  or  stop,  coincides 
with  the  optical  axis  of  the  camera  lens.  With  this  disk  there 
is  a  simple  control  mechanism  by  means  of  which  time  and  in- 
stantaneous exposures  can  be  made. 


*COMPOUND    AUTOMATIC    SHUTTER 

The  automatic  shutter  with  which  most  focusing  cameras  are 
equipped,  consists  of  a  series  of  blaflcs  which  close  to  and  open 


*Conrtesy  of  Eastman  Kodak  Co. 


Military  Topography  and  Photography 

.from  the  center.  This  center  coincides  with  the 
optical  axis  of  the  lens,  and  when  the  blades 
are  closed  they  are  light  tight.  The  blades  are 
controlled  by  a  delicate  mechanism  by  means  of 
which  the  length  of  exposure,  or  opening  at 
which  set,  is  made  very  accurate.  In  double 
combination  lenses,  the  shutter  blades  are  be- 
tween the  combinations.  The  adjustable  stop 
is  close  to  the  shutter  blades. 

The  focal-plane  shutter  consists  of  a  curtain 
containing  slits  of  different  widths.     The' ends 


167 


*FOCAL  PLANE  SHUTTER 

of  this  curtain  are  attached  to -automatic  re- 
volving cylinders.  The  different  sized  slits  in 
combination  with  different  speeds  of  the  cylin- 
ders, give  a  large  number  of  different  length 
exposures — from  time  to  as  low  as  1/1500  of  a 
second.  The  curtain  works  in  front  of  and  in 
close  proximity  to  the  dry  plate  or  film. 

CONTROL  OF  SHUTTERS.  The  simple  shutter 
is  usually  controlled  by  a  button  or  lever  which 
releases  a  spring  that  works  the  disk.  The 
automatic  shutter  is  controlled  by  air  or  by  a 
bead,  a  pressure  on  which  pushes  in  a  plunger 

*Courtesy  of  Eastman  Kodak  Co. 


*FOCAL  PLANE 
CURTAIN 


168  Military  Topography  and  Photography 

that  releases  the  mechanism  working  the  shutter  blades.  Ex- 
cept when  working  on  "T"  and  "B,"  the  blades  close  auto- 
matically at  the  period  for  which  set. 

The  mechanism  of  an  automatic  shutter  is  very  delicate  and 
it  should  never  be  oiled.  The  metal  of  which  it  is  made  will 
not  rust  and  oil  will  destroy  the  accuracy  of  its  automatic 
action. 

THE  DRY  PLATE   (FILM) 

The  dry  plate  consists  of  a  very  perfect  glass  upon  one  side 
of  which  is  a  thin  emulsion.  This  emulsion  consists  of  a  silver 
salt  and  certain  other  chemical  compounds  held  in  a  thin  gelatin 
film.  The  silver  salt  in  this  emulsion  is  sensitive  to  light;  i.  e., 
this  silver  salt  when  struck  with  light  is  changed  to  another 
form  of  a  different  chemical  composition.  This  affected  form 
of  the  silver  salt  when  treated  with  a  Developer  is  reduced  to 
metallic  silver,  but  this  developer  has  no  effect  on  the  original 
silver  salt  and  of  course  those  portions  of  the  dry  plate  which 
have  not  been  exposed  to  light.  The  amount,  or  the  depth,  of 
the  silver  salt  that  is  changed  into  this  other  form  at  any  point 
on  the  dry  plate  is  directly  proportional  to  the  amount,  or 
intensity,  of  the  light  striking  the  dry  plate  at  that  point. 
More  light  travels  from  bright  objects  than  from  dark  objects; 
therefore,  the  position  of  bright  images  on  the  dry  plate  will 
have  more  of  the  silver  salt  changed  than  the  position  of  dark 
images.  The  plate,  therefore,  after  it  shall  have  had  the  un- 
changed silver  salt  dissolved  out  of  it  by  the  Hypo- solution, 
will  appear  of  varying  degrees  of  blackness,  according  to  the 
intensity  of  the  metallic  silver  at  the  different  points  of  the 
plate.  Should  any  portion  of  the  dry  plate  be  unexposed  to 
light,  the  silver  salt  in  that  portion  will  be  entirely  dissolved 
and  washed  out,  and  that  proportion  will  be  represented  only  by 
a  transparent  gelatin  film.  The  bright  images  on  the  dry  plate 
will  appear  dark  in  the  negative,  while  dark  images  will  appear 
light — just  the  contrast  of  the  scene  photographed.  To  get 
a  corrected  view  we  must  take  a  photograph  of  the  nega- 
tive, which  is  usually  done  by  placing  a  paper  holding  on  one 
surface  a  sensitive  silver  salt,  in  contact  with  negative,  causing 
the  light  to  pass  through  the  negative  before  striking  the  silver 


Military  Topography  and  Photography  169 

salt  on  the  paper  and  thus  regulating  the  intensity  of  the  light 
and  the  amount  of  silver  salt  on  the  paper  changed  to  the  other 
form.  This  process  is  called  "Printing"  and  is  exactly  similar 
to  "Blue  Printing.!'  The  image  or  picture  developed  on  the 
paper  is  called  a  positive. 

Colors  are  not  reproduced  in  a  negative  or  print;  in  fact 
some  colors  have  no  more  effect  on  the  dry  plate  than  black 
objects.  Thus,  a  blue  flower  appears  white  while  a  yellow  one 
appears  black.  Certain  dyes  dissolved  into  the  sensitive  emul- 
sion render  certain  colors  sensitive  to  the  silver  salt,  so  that 
when  these  colors  are  photographed,  they  appear  as  "lights" 
of  varying  degrees.  Dry  plates  when  so  treated  are  called 
orthochromatic  plates,  and  such  plates  are  rapidly  replacing 
the  common  dry  plate  on  the  market. 

Plates  are  made  with  emulsions  of  varying  degrees  of  sensi- 
tiveness ;  some  plates  being  much  faster — requiring  less  expos- 
ure to  light  to  have  the  same  effect  produced  on  them.  Manu- 
factured dry  plates  are  far  superior  in  quality  and  far  cheaper 
in  price  than  home  made  plates  •  they  are  made  under  secret 
processes ;  so  that  their  preparation  need  not  be  considered 
further. 

It  should  be  observed  that  a  negative  or  photograph  is  not  a 
reproduction  of  colors,  but  of  lights  and  shadows  which  to  the 
eye  presents  a  likeness  of  the  thing  photographed.  It  is  similar 
to  a  pencil  sketch  executed  in  one  color. 

PLATE  HOLDERS.  A  plate  holder  is  a  light-tight  box  in  which 
plates  may  be  carried  in  daylight  while  not  in  the  camera 
or  original  package.  It  contains  an  opaque  partition  so  that 
one  plate  holder  may  hold  two  dry  plates  at  the  same  time.  On 
either  side  of  the  holder  is  an-  opaque  slide,  which  is  removed 
while  loading,  unloading,  or  making  an  exposure.  One  side  of 
the  slide  is  marked  "Exposed,"  and  when  the  dry  plate  has 
been  exposed  the  slide  is  inserted  with  this  side  out. 

To  load  the  plate  holder,  the  original  box  containing  the 
plates  is  opened  in  the  dark  room  which  contains  a  greatly  re- 
duced red  light.  The  slides  are  removed  fr4?m  the  holder  and 
a  plate  inserted  in  each  side.  The  emulsion  side  of  the  plate 
(does  not  reflect  light)  faces  out  and  its  surface  should  be 


170  Military  Topogravhy  mid  Photography 

brushed  with  a  soft  camel's  hair  brush  to  remove  any  dust  that 
might  be  there.  The  slides  are  then  inserted  with  the  plain 
side  out,  denoting  the  plates  to  be  "unexposcd." 

ROLL  FILMS.  A  roll  film  is  a  strip  of  transparent  film  on 
one  side  of  which  there  is  a  .sensitive  emulsion  just  the  same  as 
in  case  of  a  dry  plate.  This  rilm  is  long  enough  to  contain  from 
six  to  twelve  exposures  and  :s  wound  on  a  spool.  To  each  end 
of  the  film  is  attached  a  strip  of  opaque  paper  which  makes  the 
roll  light  tight  when  entirely  Wound  from  either  end.  These 
rolls  when  entirely  wound  can  be  loaded  into  and  removed  from 
the  camera  in  broad  daylight.  Roll  films  are  the  handiest  for 
field  work,  not  only  for  the  ease  with  which  they  can  be  inserted 
into  and  removed  from  ihe  camera  in  daylight,  but  also  from 
the  ease  with  which  they  can  be  carried  on  the  person. 

FILM  PACKS.  Film  packs  are  composed  of  12  films  of  single 
exposure  each,  held  in  a  light  tight  pasteboard  box.  This  box 
is  used  just  the  same  as  a  plate  holder.  The  films  can  be  ex- 
posed in  succession  and  the  box  in  daylight.  It  is  superior 


*FILM  PACK  ADAPTER 

to  a  roll  .film  in  that  the  portion  of  exposed  film  may  be  re- 
moved at  any  time  in  a  dark  room  and  the  box  be  used  again 
for  the  unexposed  films,  while  a  roll  film  must  have  all  of  its 
exposures  made  before  being  removed  from  the  camera  if  it  is 
desired  to  use  each  exposure. 

t      STYLES   OF   CAMERAS 

The  different  styles  of  cameras  that  might  be  used  in  military 
operations  are — the  box  camera,  the  folding  camera,  the  view 

*Courtesy  of  Eastman  Kodak  Co. 


Military  Topograplvy  and  Photography 


171 


camera,  the  graflex  camera,  and  the  enlarging  camera.  No 
description  of  these  cameras  is  necessary  for  all  are  familiar 
with  them  in  a  general  way. 


*VEST  POCKET  KODAK 

For  reconnaissance  work  the  "Vest  Pocket  Kodak"  and  the 
"No.  0  Graphic  Camera,"  manufactured  by  the  Eastman  Kodak 
Co.,  are  the  most  compact  and  easily  carried.  They  are  sus- 
ceptible of  excellent  work,  but  when  used  an  enlarging  camera 
must  also  be  employed  to  enlarge  the  print  to  a  proper  size. 


'No.  0  GRAPHIC  CAMERA 


If  it  is  practicable  to  carry  a  larger  camera,  the  3A  Kodak 
equipped  with  the  Zeiss  Kodak  Lens,  or  a  3A  Graflex  with  a 
B.  &  L.  Zeiss  Tessar,  Ser.  Ic.  lens  (manufactured  by  the  East- 


*0ourtesy  of  Eastman   Kodak  Co. 


172 


Military  Topography  and  Photography 


*VIEW  CAMERA 


man  Kodak  Co.)  should  be  used  Where  position  views  are 
desired  near  the  vicinity  of  troops,  a  larger  camera  can  be  used, 
such  as  the  view  or  panoramic  camera. 


*  PRINCIPLE  OF  ENLARGEMENT 

USING  THE  CAMERA 

POSITION  FOR  EXPOSURE.  No  matter  what  the  vertical  angle 
of  the  optical  axis,  or  position  of  the  lens,  the  camera  should 
be  so  held  or  placed  that  the  ground  glass  is  exactly  parallel 
with  the  plane  of  the  perspective.  If  the  camera  is  held  so 
that  the  ground  glass  is  not  parallel  with  the  plane  of  the  per- 
spective, the  object  photographed  will  be  distorted  in  the  pic- 
ture according  to  the  degree  which  the  background  is  out  of 
the  parallel. 

*  Courtesy  of  Eastman  Kodak  Co. 


Military  Topogrcphy  and  Photography 


173 


JU 


^DISTORTED  PICTURE 

SUPPORT  FOR  EXPOSURE.  For  instantaneous  exposures  the 
camera  may  be  held  in  the  hand,  but  for  timed  exposures  the 
camera  must  be  supported  on  a  tripod,  table,  or  other  solid 
support. 

ADJUSTMENTS  FOR  EXPOSURES.  Bearing  in  mind  that  the 
plane  of  the  background  must  be  vertical  as  explained  above,  the 
optical  axis  of  the  camera  must  be  brought  into  the  azimuth  of 
the  object  to  the  photographed.  When  it  is  brought  into  the 
proper  horizontal  position  on  the  ground  glass,  as  indicated  by 
the  view  finder,  or  as  shown  on  the  ground  glass  itself,  the 
camera  box  should  be  held  in  place  and  other  adjustments  used 
to  brin^f  the  image  into  the  proper  vertical  position  on  the 
ground  glass.  If  the  object  to  be  photographed  is  high,  such 
as  a  near  building,  and  the  photographer  cannot  get  far  enough 
away  on  account  of  surrounding  buildings,  etc.,  the  camera  base 
should  be  tilted  upward  so  that  the  object  falls  entirely  on  the 
ground  glass,  the  camera  back  being  kept  vertical.  If  it  is 
desired  to  make  only  a  small  change  in  the  vertical  position  of 
the  image  on  the  ground  glass,  it  can  be  accomplished  by  rais- 
ing or  lowering  the  lens  on  its  support,  the  required  amount. 

*Courtesy  of  Eastman  Kodak  Co, 


174  Military  Topography  and  Photography 

FOCUSING — Judging  the  Distance.  When  an  object  is  less 
than  100  feet  away,  and  the  camera  used  has  no  ground  glass  on 
which  the  image  can  be  focused,  the  distances  must  be  estimated 
and  the  closer  the  object  to  the  camera  the  more  accurate  the 
estimate  must  be.  The  photographer  should,  therefore,  have  a 
good  idea  of  distances — ten,  fifteen,  twenty-five,  fifty,  seventy- 
five,  and  one  hundred  feet,  should  be  definite  conceptions  in  his 
mind.  Knowing  these  distances,  he  can  bisect  them  to  obtain 
other  ranges.  Thus,  an  object  might  be  half  way  between  15 
and  25  feet,  or  just  twice  ten  feet  away,  from  which  the  photog- 
rapher could  make  an  intelligent  estimate  of  20  feet. 

TIMING  THE  EXPOSURE.  The  time  of  the  exposure  will  depend 
upon  the  intensity  of  the  light  and  the  size  of  stop  used.  In 
order  to  make  accurate  estimates  of  the  time  required,  the 
photographer  must  learn  from  experience.  Between  three 
hours  after  sunrise  and  three  hours  before  sunset,  the  following 
are  given  for  the  northern  United  States,  when  using  the  f.128 
stop  : 

With  Sunshine 1/100  of  a  second 

With  Light  Clouds 1/2  to  1  second 

With  Heavy  Clouds 2  to  5  seconds 

With  a  good  lens,  an  f.16,  or  f.32  stop  is  recommended.  If 
in  a  given  condition  of  light,  one  second  is  the  proper  time  for 
the  f.16  stop,  then  four  seconds  is  the  proper  time  if  the  shutter 
is  closed  to  f.32. 

DEVELOPING 

THE  DARK  ROOM.  The  dark  room  should  be  a  light-tight 
room  of  sufficient  size  to  carry  on  the  work  of  developing 
properly.  It  is  best  to  paper  the  walls  with  black  paper.  The 
room  should  be  provided  with  a  light-tight  ventilating  flue.  In 
the  field,  in  the  theater  of  operations,  where  a  dark  room  cannot 
be  improvised,  a  tent  can  be  made  into  a  dark  room.  Such  a 
tent  should  be  painted  black,  or  otherwise  rendered  entirely 
opaque. 

LIGHTING  THE  DARK  ROOM.  The  dark  room  may  be  illumin- 
ated with  a  dark  room  lantern  or  a  small  colored  window.  The 
dark  lantern  should  be  provided  with  two  or  more  red  colored 
glass  slides  to  control  the  intensity  of  the  light,  and  a  cover  or 


Military  Topography  and  Photography 


175 


lid  to  shut  the  light  off  entirely.  The  intensity  of  the  light  from 
a  red  colored  window  glass  can  be  controlled  by  placing  one  or 
more  covers  of  red  paper  over  it.  It  should  also  be  provided 
with  a  door  or  black  curtain  to  shut  the  light  out  entirely. 

WATER  SUPPLY.  If  possible  the  dark  room  should  be  plumbed 
for  running  water.  A  water  supply  or  pressure  tank  can  be 
used  to  great  advantage  where  a  city  supply  is  not  available. 
If  neither  of  these  sources  is  available,  the  water  will  have  to 
be  carried  from  a  well — clear  rain  water  should  be  used  if 
available.  Two  galvanized  iron  tubs  will  suffice  for  washing 
plates  and  prints,  transferring  frequently  from  one  tub  to  the 
other. 


Wall 


Ped  Light  / 

!k — L— 


FIG.  66 

ARRANGEMENT  OF  APPARATUS.  The  developing  bench  should 
be  somewhat  similar  to  the  sink  in  a  kitchen.  In  the  center 
Ilirrr  should  be  a  basin  with  drain  and  a  water  supply  spigot 
above  it;  on  either  side  there  should  be  corrugated  surfaces 
on  which  to  place  the  developing  and  fixing  trays.  The  ridges 
of  this  surface  should  be  level;  the  troughs  should  slope  towards 
the  basin  to  furnish  drainage.  For  developing  there  should 
always  be  three  trays  arranged  in  series:  first,  the  tray  con- 
taining the  developer;  second,  a  tray  with  pure  water;  and 
third,  a  tray  containing  the  fixing  bath,  or  hypo.  Opposite  the 
developing  tray  there  should  be  a  small  red  colored  window,  or 
a  dark  lantern,  in  order  to  see  the  progress  and  the  completion 
of  the  development.  See  Fig.  66. 

Above  and  below  the  developing  bench,  there  should  be  shelves 
so  that  any  apparatus  or  supplies  that  may  be  required  during 


176  Military  Topography  and  Photography 

the  developing  shall  be  easily  available  and  be  out  of  the  way 
when  not  in  use.  The  developing  bench  should  of  course  be  next 
to  the  wall,  and  in  northern  latitudes  against  the  north  wall. 
The  door  to  the  dark  room  should  be  within  the  building,  and 
when  closed  light  tight. 

CLEANLINESS.  The  trays,  the  developing  bench,  and  the 
whole  dark  room  should  be  absolutely  clean.  Care  must  always 
be  taken  that  not  a  trace  of  hypo  ever  gets  into  the  developer. 
Whenever  the  fingers  are  put  in  the  hypo,  they  should  be  im- 
mediately rinsed  off  in  pure  water  and  wiped.  No  dirt  or 
foreign  chemicals  should  ever  get  into  the  developer  and  fixing 
bath,  and  when  they  do,  the  solution  should  be  thrown  away  and 
a  new  one  prepared.  Good  negatives  and  pictures  cannot  be 
produced  where  there  are  dirty  or  impure  developing  and  fixing 
solutions,  and  too  much  care  cannot  be  taken  to  prevent  any 
and  all  uncleanliness. 

THE  DEVELOPER.  The  developer  is  a  solution  of  several 
chemicals  used  to  develop  a  dry  plate  after  it  has  been  exposed 
to  light.  The  development  of  a  plate  is  the  reduction  to  metallic 
silver  of  that  portion  of  the  sensitive  silver  salt  that  has  been 
changed  by  the  effect  of  light.  The  amount  of  silver  salt  that 
is  capable  of  being  reduced  to  metallic  silver  at  any  place  on 
the  plate  depends  upon  the  ambunt  of  light  that  struck  that 
place  during  exposure ;  and  the  reduction  of  the  affected  silver 
salt  may  be  stopped  at  any  time  by  removing  the  plate  from 
the  developer.  The  photographer  is  thus  able  to  develop  an 
exposed  dry  plate  to  any  density  that  he  may  wish,  provided 
of  course  the  plate  has  not  been  underexposed.  The  chemical 
which  reduces  the  affected  silver  salt  is  called  the  active  or 
developing  agent.  There  are  many  developing  agents  on  the 
market,  but  the  Pyro  and  Hydrochinon-Metol  Developers  are 
the  best  known  to  amateurs. 

The  developing  agent  alone  reduces  the  affected  silver  salt 
very  slowly,  but  when  an  alkali  is  mixed  with  it,  the  developing 
agent  has  a  greater  affinity  for  oxygen  and  therefore  becomes 
quite  energetic  in  its  reduction  of  the  affected  silver  salt.  The 
alkalies  most  commonly  used  are  Sodium  Carbonate  and  Potas- 
sium Carbonate.  The  alkali  is  called  an  accelerator. 


Military  Topography  and  Photography  177 

Often  the  development  of  a  particular  exposed  plate  is  too 
energetic.  In  such  cases  the  development  can  be  retarded  by 
the  addition  of  Potassium  Bromide,  or  other  restrainer.  The 
action  of  the  Potassium  Bromide  is  to  dissolve  some  of  the  silver 
salt  (Silver  Bromide)  out  of  the  emulsion,  thereby  forming  a 
double  salt  of  silver  which  is  less  easily  reduced  to  metallic 
silver  by  the  developing  agent. 

When  it  is  desired  to  keep  the  developer  from  discoloring  and 
oxidizing  a  preservative  is  added  to  it.  The  preservative  most 
commonly  used  is  Sodium  Sulphite.  In  addition  to  its  qualities 
as  a  preservative,  Sodium  Sulphite  has  much  to  do  with  the 
color  of  the  negative.  If  only  a  small  amount  is  used  the 
negative  will  be  brown  in  color ^and  the  quality  harsh  and  hard, 
while  a  greater  portion  will  give  a  gray,  soft  negative  with 
more  detail. 

The  developer  may  be  in  the  form  of  prepared  powder,  in 
which  case  it  is  only  necessary  to  dissolve  it  in  the  amount  of 
water  prescribed  in  the  directions  accompanying  it.  If  in  a 
concentrated  liquid  form,  it  must  be  diluted.  If  one  makes 
one's  own  developer,  the  chemicals  should  be  accurately  weighed 
and  used  in  the  proportion  prescribed  in  the  formula. 

PYKO  DEVELOPING  FORMULA* 

Pyrogallic  Acid  Solution 

"A"                                                    Avoirdupois  Metric  System 

Pyrogallic  Acid                                                    1  oz.  30  grams 

Sulphuric  Acid                                                     20  minims.  1  c.      c. 

Water                                                                     28  ozs.  900  c.      c. 
("B")     Soda  Solution 

Carbonate  Soda    (desiccatedf)                           2  ozs.  60  grams 

Sulphite  Soda  (desiccatedf)                               3  ozs.  90  grams 

Water                                                                   28  ozs.  900  c.      c. 

For  Dark-Room  Development  Take 

"A"  i/2  oz.  15  c.      c. 

"B"  1/3  oz.  15  c.      c. 

Water  4  ozs.  120  c.      c. 

This  developer  will  then  contain  1.56  grains  Pyro  per  oz. 

*Page  97,  How  to  Make  Good  Pictures,  Eastman   Kodak   Co. 
flf  crystals  are  used,  double  the  quantity. 


178 


Military  Topography  and  Photography 


ELOK-HYDROCIIINOK  OR  METOL-HYDROCiuisroisr* 

Solution  A 
Elon  or  Metol 
Hydrochinon 

Sulphite  Soda   (dessicated) 
Water 

Solution  B 

Carbonate  Soda  (dessicated) 
Water 


00  grains 
30  grains 
%  oz. 
20  ozs. 


Y»  oz. 
20  ozs. 


4  grams 

2  grams 

22.5  grams 

600  c.      c. 


15  grams 
600  c.      c. 


For  Dark-Room  Development  Take 

Solution  "B"  1  oz.  30  c.      c. 

Solution  "B"  1  oz.  30  c.      c. 

Water  2  ozs.  60  c.      c. 

Potassium  Bromide,  10%  4  to  8  drops 

DEVELOPING.  Normal  Procedure :  The  plate  to  be  developed 
is  removed  from  the  plate  holder  after  exposure  in  the  dark 
room.  Its  emulsion  surface  is  brushed  off  with  a  fine  camel 
hair  brush  to  remove  any  traces  of  dust  that  may  be  adhering 
to  it.  The  plate  is  then  immersed  into  the  developing  solution, 
emulsion  side  up.  The  solution  should  cover  the  plate  as  evenly 
and  quickly  as  possible;  all  air  bubbles  forming  on  the  surface 
should  be  removed  at  once  with  a  light  brush  of  the  fingers. 
The  tray  should  be  rocked  back  and  forth  so  as  to  keep  the 


FILM  PACK  TANK 


developer  in  contact  with  the  plate  in  constant  motion.  The 
developer  should  be  from  65°  to  70°  F.  If  the  red  light  (should 
be  greatly  reduced)  from  the  dark  lantern  be  cast  upon  the 
plate,  the  high  lights  will  be  seen  to  appear  first  and  then  a 
well  defined  image.  The  development  is  not  yet  complete:  it 

*Page  97,  How  To  Make  Good  Pictures,  Eastman  Kodak  Co. 
fCourtesy   of   Eastman    Kodak   Co. 


Military  Topography  and  Photography  179 

must  be  carried  on  until  the  negative  is  very  dense.  The  fixing 
of  the  plate  will  greatly  reduce  this  density,  and  the  resulting 
image  after  the  fixing  will  he  clear  and  distinct.  When  the 
development  has  proceeded  to  the  proper  point  as  explained, 
the  negative  is  removed  from  the  developer,  rinsed  off  in  a  tray 
f  mi  re  water,  and  then  immersed  in  the  hypo,  or  fixing  tray. 

Overexposed  Plates:  Should  the  image  come  out  very  quick- 
ly instead  of  slowly  and  regularly,  the  plate  has  been  over- 
exposed. In  such  cases  remove  the  plate  from  the  developer 
at  once  and  place  it  in  the  tray  containing  the  pure  water;  add 
a  few  drops,  according  to  .the  degree  of  overexposure,  of 
Potassium  Bromide  Solution  (45  grains  of  KBr  to  one  ounce 
of  water)  to  the  developer.  Put  the  plate  back  into  the  de- 
veloper and  proceed  as  before. 

Underexposed  Plates:  If  the  plate  develops  slowly,  no  de- 
tails appear  in  the  shadows,  but  high  lights  come  up  quickly, 
the  plate  has  been  underexposed.  In  such  cases  remove  the 
plate  from  the  developer  and  put  it  into  the  tray  containing  the 
pure  water ;  dilute  the  developer  with  an  equal  amount  of  pure 
water  (o¥  the  same  temperature)  ;  put  the  plate  back  into  the 
developer  and  proceed  as  before. 

The  developer  should  never  be  diluted  or  have  a  chemical 
added  to  it  while  tlie  plate  is  in  it,  as  the  resulting  solution  will 
not  be  of  uniform  strength  quickly  enough.  Such  changes  in 
the  strength  of  the  developer  should  be  made  only  after  the 
plate  has  been  removed  from  it,  and  the  plate  should  not  be  put 
back  until  the  added  chemical  has  had  time  to  mix  thoroughly. 

DAYLIGHT  DEVELOPMENT.  By  the  use  of  the  film  tank  de- 
veloper roll  films  can  be  developed  in  broad  daylight.  The  tank 
is  loaded  with  the  film  much  the  same  as  a  roll  film  camera  is. 
In  the  use  of  these  tanks,  the  kind  and  strength  of  developer 
recommended  by  the  makers,  should  always  be  used,  and  the 
time  specified  for  the  developing  rigidly  followed.  The  film 
tank  developer  permits  developing  in  the  field  without  other 
accessories,  where  a  dark  room  is  not  usually  available,  and 
its  use  commends  itself  for  military  purposes. 

For  field  purposes  prepared  developing  powders  should  be 
used.  They  eliminate  the  necessity  of  weighing  and  mixing, 
and  besides  are  more  conveniently  transported. 


180  Military  Topography  and  Photography 

THE   FOLLOWING   INSTRUCTIONS   ARE   GIVEN   BY  THE   EASTMAN   KODAK   Co., 

FOR  THE   USE  OF  THEIR  KODAK  FlLM  TANK* 

"The  Kodak  Film  Tank  consists  of  a  wooden  box,  a  light-proof  apron, 
a  'Transferring  Reel,'  a  metal  'solution  cup,'  in  which  the  film  is  developed, 
and  a  hooked  rod  for  removing  film  from  solution.  There  is  also  a  dummy 
film  cartridge  with  which  one  should  experiment  before  using  an  exposed 
cartridge.  The  various  parts  of  the  outfit  come  packed  in  the  box  itself. 

"1.     Take  everything  out  of  the  box.    Take  the  apron 
Setting  up  and  Transferring  Reel  out  of  the  solution  cup. 

the  "2.     The  axles  marked  C  and  D  in  the  cut  (Fig.  67) 

Film  Tank:  are  to  be  inserted  in  the  holes  in  the  front  of  the  box. 

The  front  will  be  toward  you  when  the  spool  carrier  in 
end  of  box  is  at  your  right.  These  axles  are  interchangeable.  The  axle  *C* 
must  be  pushed  through  the  hollow  spindle  which  will  be  found  loose  in  the 
box.  The  spindle  has  a  lug  at  each  end  to  which  the  hooks  of  the  apron  are 
to  be  attached. 

"3.  The  axle  'D'  must  be  pushed  through  the  hollow  rod  of  the  Trans- 
ferring Reel  in  position  as  indicated  in  the  illustration.  The  flanges  at  each 


end  of  the  Transferring  Reel  are  marked  'Y'  in  the  illustration  (Fig.  67). 
Both  axles  'C'  and  'D'  must  be  pushed  clear  through  into  the  holes  on  the 
opposite  side  of  the  box. 

"4.  Attach  one  end  of  the  apron  to  spindle,  through  which  axle  'C' 
passes,  by  means  of  the  metal  hooks  which  are  to  be  engaged  with  lugs  on 
the  spindle  (Fig.  68).  The  corrugated  side  of  the  rubber  bands  is  to  be 
beneath  the  apron  when  it  is  attached.  Turn  to  the  left  on  axle  'C'  and  wind 
entire  apron  onto  axle,  maintaining  a  slight  tension  on  apron,  in  so  doing, 
by  resting  one  hand  on  it. 

"5.  Insert  film  cartridge  in  spool  carrier  (Fig.  69),  and  close  up  the 
movable  arm  tight  against  end  of  spool.  Have  the  duplex  paper  ('B'  in 
Fig.  67)  lead  from  the  top. 

"Film  to  be  used  in  the  Kodak  Film  Tank  must  be  fastened  to  the  duplex 

paper  at  both  ends.    All  Kodak  films  are  fastened  at  one  end 

Important:         in  the  factory.     The  other  end  is  fastened  in  the  following 

manner.     Just   before  you  are  ready   to  develop    (holding 

spool  with  the  black  side  of  the  duplex  paper  up)  unroll  the  duplex  paper 

carefully  until  you  uncover  the  piece  of  gummed  paper  which  is  fastened  to 

*How  To  Make,  Good  Pictures,  Eastman  Kodak  Co, 
f Courtesy  of  Eastman  Kodak  Co. 


*Fio.   68 


*Fio.  69 
*Courtesy  of  Eastman  Kodak  Co. 


182 


Military  Topography  and  Photography 


end  of  film  and  is  to  be  used  as  a  means  of  fastening  film  to  duplex  paper. 
Moisten  the  gummed  side  of  sticker  evenly  for  about  an  inch  across  the  end 
and  stick  it  down  to  duplex  paper,  rubbing  thoroughly  to  secure  perfect 
adhesion.  Wind  end  of  duplex  paper  on  spool  again^and  the  cartridge  is 
ready  to  insert  in  machine. 


"6.  Break  the  sticker  that  holds  down  the  end  of  duplex  paper,  thread 
the  paper  underneath  wire  guard  on  Transferring  Reel — through  which  axle 
'D'  passes  (Fig.  70),  and  turn  axle  slowly  to  right  until  the  word  'stop' 
appears  on  duplex  paper. 


71 


*Courtesy  of  Eastman  Kodak  Co. 


Military  Topography  and  Photography  183 

"  "7.  Now  Hook  apron  to  lugs  on  Transferring  Reel  (Fig.  71),  in  precisely 
the  same  manner  that  you  hooked  the  opposite  end  in  lugs  on  spindle, 
except  that  axle  'D'  turns  to  the  right. 

"8.  Turn  handle  half  a  revolution  so  that  apron  becomes  firmly  attached 
and  put  cover  on  box.  Turn  axle  'D'  slowly  and  steadily  until  duplex 
paper,  film,  and  apron  are  rolled  up  together  on  Reel.  As  soon  as  this  is 
completed  the  handle  will  turn  very  freely." 

9.  Put  four  or  five  ounces  of  lukewarm  water  into  the  Solution  Cup 
and  dissolve  in  it  the  following  chemicals  in  the  order  named  (for  5"  and 
7"  tanks): 

30  grains  Pyro 

60  gains  Sulphite  of  Soda  (desiccated) 
60  grains  Carbonate  of  Soda  (desiccated) 

or  better,  in  place  of  these  chemicals,  "dissolve  in  it  the  contents  of  the  large 
package  of  the  Kodak  Tank  Developer  Powders,  and  fill  the  cup  with  cold 
water  to  the  embossed  ring — not  to  Uie  top.  In  the  latter  case,  dissolve  the 
contents  of  the  small  package  in  the  solution  and  the  developer  will  be  ready: 
in  the  former,  fill  the  tank  to  the  embossed  ring — not  to  the  top,  with  cold 
water.  The  temperature  of  the  developer  should  be  65°  Fahr."  "For  Brownie 
Tanks  use  ^  the  amounts  above  specified:  for  the  3}£"  Tank  use  11/15." 


*Fio.   72 

"10.  Now  remove  cover  from  box  and  draw  out  axle  'D'  (Fig.  72), 
holding  apron  and  duplex  paper  with  other  hand  to  keep  end"  of  apron,  from 
loosening. 

"11.  Remove  entire  Transferring  Reel  (now  containing  apron,  duplex 
paper,  and  film)  which  is  freed  by  pulling  out  axle  D,  and  insert  imme- 
diately in  the  previously  prepared  developer. 

"In  removing  Reel  do  not  squeeze  the  apron,  but  hold  it  loosely  or  slip 
a  rubber  band  around  it  to  keep  from  unrolling. 

"12.  *  *  *  Lower  Transferring  Reel  into  Cup  (Fig.  73),  with  the 
end  containing  crossbar  up.  Let  Reel  slide  down  slowly.  The  operation 
of  removing  reel  from  box  can  be  done  in  the  light  of  an  ordinary  room, 

*Courtesy  of  Eastman   Kodak  Co. 


184 


Military  Topography  and  Photography 


*FIG.  73 


*Fio.  74 


but  for  safety  it  is  well  that  the  light  should  not  be  too  bright.     The  total 
length  of  time  for  development  is  20  minutes. 

"NOTE:  Immediately  after  lowering  Reel  into  solution  cup,  catch  it  with  wire  hook 
and  move  slowly  up  and  down  two  or  three  times,  taking  care,  however,  not  to  raise 
any  part  of  Reel  above  the  surface  of  the  solution.  This  is  to  expel  air  bubbles. 

"13.  Then  place  the  cover  on  the  cup  (Fig.  74)  putting  lugs  on  cover  into 
the  grooves  and  tighten  cover  down  by  turning  to  right.  Now  turn  the 
entire  cup  end  for  end,  and  place  in  a  tray  or  saucer  to  catch  any  slight 
leak  in  the  cup.  At  the  end  of  three  minutes  reverse  the  cup,  and,  there- 
after, reverse  every  three  minutes  until  the  time  of  development  (20 
minutes)  has  elapsed.  Turning  the  solution  cup  in  this  manner  allows  the 
developer  to  act  evenly  and  adds  brilliancy  and  snap  to  the  negatives.  The 
wire  hook  is  to  be  used  for  lifting  the  reel  out  of  cup.  Hook  on  to  cross- 
bar in  one  end  of  reel  (Fig.  75). 


*Fio.  75 


*Fio.  76 


*Courtesy  of  Eastman  Kodak  Co. 


Military  Topography  and  Photography  185 

"14.  When  development  is  completed  pour  out  developer  and  fill  cup 
with  clear,  cold  water  and  pour  off,  repeating  this  operation  three  times  to 
wash  the  film.  Then  remove  Transferring  Reel;  separate  film  from  duplex 
paper  and  place  immediately  in  the  Fixing  Bath,  which  should  be  in 
readiness  as  explained  in  15.* 

"The  film  may  be  separated  from  duplex  paper  in  light  of  an  ordinary 
room,  if  the  developer  is  thoroughly  washed  out.  The  operation  of  separating 
film  and  duplex  paper  should  be  done  over  a  bowl,  bath  tub,  or  sink.  When 
the  duplex  paper  does  not  free  itself  readily  from  back  of  film,  split  the 
paper  where  possible,  this  will  remove  the  hard  outer  surface  of  the  paper, 
the  remaining  portion  will  soon  become  soaked  and  then  can  be  removed 
easily  by  rubbing  gently,  while  immersed,  with  the  ball  of  the  finger.  This 
adhering  of  the  duplex  paper  to  the  film  is  almost  invariably  caused  by  the 
use  of  too  warm  developer." 

FIXING 

"15.  Provide  a  box  of  Kodak  Acid  Fixing  Powder  which  should  be  pre- 
pared as  per  instructions  on  the  package.  Put  this  into  a 
The  Fixing  tray  or  wash  bowl-.  When  the  powder  is  thoroughly  dis- 

Bath:  solved  add  to  the  solution  as  much  of  the  Acidifier,  which  you 

will  find  in  a  small  box  inside  the  large  one,  as  directions  call 

for.    As  soon  as  this  has  dissolved  the  Fixing  Bath  is  ready  for  use.    Any 

quantity  of  the  bath  may  be  prepared  in  the  above  proportions.    The  Fixing 

Bath  given  on  the  next  page  may  be  used." 

"16.     Pass  the  film  face  down   (the  face  is  the  dull  side)   through  the 

fixing  solution  as  shown  in  the  cut  (Fig.  76),  holding  one  end  in  each  hand. 

Do  this  three  or  four  times  and  then  place  one  end  of  the  film  in  the  tray, 

(8"  X  10"  is  a  good  size)  still  face  down,  and  lower  the  strip  into 

Fixing:    the  solution  in  folds.    Gently  press  the  film  where  the  folds  occur, 

not  tightly  enough  to  crack  it,  down  into  the  solution  during  the 

course  of  fixing.    This  insures  the  fixing  solution  reaching  every  part  of  the 

film.     Allow  the  film  to  remain  in  the  solution  two  or  three  minutes  after 

it  has  cleared,  or  the  milky   appearance  has  disappeared.     Then  remove 

for  washing. 

"NOTE:    If  preferred,  negatives  may  be  cut  apart  and  fixed  separately." 

"After  developing  a  roll  of  film  the  apron  must  be  wiped  dry  before 
developing  another  roll.  The  apron  will  dry  almost  instantly  if  immersed 
for  a  moment  in  very  hot  water.  Keep  apron  wound  on  axle  'D'  when  not 
in  use.  Never  leave  apron  soaking  in  water." 

"Several  rolls  of  film  may  be  developed  at  the  same  time  if  the  operator 
wishes.  To  do  this  it  is  necessary  to  have  a  'Duplicating  Outfit'  consisting 
of  1  Solution  Cup  and  cover,  1  Transferring  Reel,  and  1  Apron  for  each 
additional  roll  of  film  to  be  developed.  The  extra  rolls  of  film  may  then 
be  wound  on  to  Transferring  Reels  as  previously  described  and  immersed 
into  the  Solution  Cups."* 

If  it  is  desired  to  develop  twice  as  fast  double  strength  developer  is  used. 
Should  the  normal  of  65°  Fahr.  be  impossible  to  maintain,  the  following 

*The  liberty  of  slightly  rearranging  the  above  instructions  has  been  taken  in 
order  to  avoid  the  cross  references  of  the  text. 


186  Military  Topography  and  Photography 

table*  may  be  used  for  obtaining  the  time  of  development,  interpolating  the 
intermediate  values. 

Time  Time 
Temperature 

One  Powder  Two  Powders 

70°                                         15  minutes  8  minutes 

65°          NORMAL            20  minutes  10  minutes 

60°         —                             25  minutes  11  minutes 

55°         —                             30  minutes  13  minutes 

50°                                         35  minutes  15  minutes 

45°         —                             40  minutes  17  minutes 

THE  FIXING  BATH  AND  FIXING.  After  the  negative  has  been 
developed  to  the  point  desired,  removed  from  the  developer, 
and  rinsed  off  in  pure  water,  it  is  then  immersed  into  a  solution 
of  Hyposulphite  of  Soda  which  dissolves  out  of  the  film  all  the 
silver  salt  that  has  not  been  affected  by  light.  Until  the  nega- 
tive has  been  immersed  into  the  Hyposulphite  solution,  it  is 
sensitive  to  light  and  all  the  operations  up  to  this  point  must 
be  carried  on  in  the  dark  room;  from  now  on  the  negative  is 
insensitive  to  light.  The  negative,  however,  is  kept  in  the 
Hyposulphite,  or  Hypo  Solution,  as  it  is  commonly  called,  until 
after  all  the  silver  salt  has  been  dissolved  out.  When  the  silver 
salt  is  completely  dissolved  out  of  the  negative,  all  the  creamy 
appearance  will  have  left  the  back  of  the  negative:  it  is  then 
"fixed."  This  "fixing"  must  be  complete  to  preserve  permanent- 
ly the  negative.  In  order  to  make  the  film  surface  of  the  nega- 
tive hard  and  tough  to  withstand  handling,  a  hardening  solu- 
tion is  added  to  the  Hypo. 

FIXING  BATH1 

Water  16  ozs.  480  c.      c. 

Hyposulphite  of  Soda  4  ozs.  120  grams 

Sulphite  of  Soda  (desiccated)  %  oz.  7.5  grams 
When  fully  dissolved,  add  the  following  hardener: 

Powdered  Alum  %  oz.  3.75  grams 

Citric  Acid  ys  oz.  3.75  c.     c. 

This  bath  may  be  made  up  at  any  time  in  advance  and  be  used 
so  long  as  it  retains  its  strength,  or  is  not  sufficiently  dis- 
colored by  developer  carried  into  it  to  stain  the  negatives. 

For  the  same  reason  as  in  case  of  developers,  it  is  recom- 
mended that  prepared  fixing  powders  be  used,  and  especially 
so  in  field  operations. 

*Extract  from  page  86,  How  To  Make  Good  Pictures. 

1  Page  86,  How  To  Make  Good  Pictures,  Eastman  Kodak  Co. 


Military  Topography  and  Photography  187 

WASHING  AND  DRYING.  In  order  to  preserve  the  negative, 
all  the  hypo  must  be  thoroughly  washed  out  of  it.  After  having 
been  thoroughly  fixed,  the  plate  should  be  rinsed  off  in  water, 
placed  in  a  zinc  washing  box,  and  immersed  in  running  water 
for  about  thirty  minutes.  They  may  be  placed,  standing  on 
edge,  in  a  bucket  of  water,  and  have  the  water  on  them  changed 
ten  or  twelve  times,  allowing  each  change  of  water  to  remain 
on  them  for  about  -five  minutes. 

After  the  negative  has  been  thoroughly  washed,  it  should  be 
rinsed  off  in  fresh  water  and  placed  in  a  drying  rack  to  dry. 
Where  several  plates  are  developed  consecutively,  the  fixing  and 
drying  can  be  carried  on  simultaneously  and  at  a  great  saving 
of  labor. 

PRINTING 

PRINTING  PAPERS.  There  are  many  different  kinds  of  print- 
ing papers  on  the  market,  but  all  may  be  grouped  into  two 
general  classes — (1)  "printing  out"  papers,  and  (2)  "develop- 
ing out"  papers.  In  the  former,  the  picture  is  printed  on  the 
paper  as  it  is  exposed  through  the  negative  to  light ;  in  the  lat- 
ter, the  paper  is  also  exposed  to  light  through  the  negative,  but 
the  picture  does  not  show  until  it  is  developed  out.  Of  the  latter 
papers,  Yelox  and  Azo  are  the  best.  The  great  advantage  of 
these  papers  are,  they  may  be  handled  in  subdued  daylight,  take 
only  a  short  timed  exposure,  and  develop  quickly.  Velox  is 
furnished  in  two  grades,  Regular  and  Special,  and  of  different 
surfaces.  Regular  Velox  develops  quickly,  and  should  be  used 
with  thin  or  weak  negatives.  Special  Velox  develops  slowly 
and  is  intended  for  contrasty  or  dense  negatives.  For  views  of 
the  terrain,  etc.,  cither  Velox  or  Azo,  or  any  other  good  develop- 
ing out  paper  is  recommended. 

For  military  passes,  safe-guards,  etc.,  the  photo  of  the  per- 
son or  building  whose  identity  is  essential  should  be  printed 
directly  on  the  printed  blank  form.  For  this  purpose,  that 
portion  of  the  blank  form  that  is  to  contain  the  photo  should 
be  covered  with  a  film  of  emulsion  containing  a  sensitive  silver 
salt;  the  unexposed  blank  forms  should  of  course  be  kept  in 
light-tight  packages.  Passes  and  safe-guards  with  the  photo 


188  Military  Topography  and  Photography 

printed  directly  upon  them  are  much  more  difficult  to  forge 
than  those  on  which  a  photo  print  is  pasted  or  mounted. 

In  case  such  forms  are  not  furnished,  and  it  is  desired  to 
improvise  some,  the  following  method  of  preparation  may  be 
used.  The  blank  form  should  be  printed  on  a  good  quality 
note  paper,  not  too  rough.  The  following  solutions  are  made: 

SOLUTION  A 

Sodium  Phosphate                                           16  grams  4  grams 

Distilled  Water                                              14  oz.  7  cu.  cm. 

SOLUTION  B 

Silver  Nitrate                                                  20  grams  4.5  grams 

Distilled  Water                                             14  oz.  7  cu.  cm. 

SOLUTION  C 

Citric  Acid                                                         1  oz.  30  cu.  cm. 

Distilled  Water                                                  4  ozs.  120  cu.  cm. 

Mix  solutions  A  and  B  together,  in  a  dark  room  with  arti- 
ficial light  (red)  ;  add  solution  C  by  degrees  until  all  the  yellow 
silver  phosphate  dissolves.  Using  a  fine  camel  hair  brush, 
brush  the  resulting  solution  over  that  portion  of  the  form  de- 
voted to  the  picture  and  hang  it  up  in  the  dark  to  dry.  When 
dry,  this  makes  a  printing  out  paper,  which  when  exposed 
through  the  negative  gives  a  rich  reddish  brown  image.  The 
image  thus  produced  is  fixed  in  the  following  hypo  bath : 

Hyposulphite  of  Soda 1  oz. 

Water 16  ozs. 

After  having  teen  fixed,  thoroughly  wash  and  dry. 

Much  better  results  can  be  obtained  by  using  "developing 
out"  printed  blank  forms,  prepared  by  manufacturers  of  print- 
ing papers. 

PRINTING  LIGHT  AND  EXPOSURE.  For  exposing  prints  when 
Velox  or  Azo  paper  is  used,  either  daylight  or  artificial  light 
may  be  used.  If  daylight  is  used,  the  exposing  window  should 
be  on  the  north  side  of  the  dark  room,  as  the  light  on  that  side 
will  be  more  uniform.  To  subdue  this  light,  several  thicknesses 
of  white  tissue  paper  may  be  put  over  this  window,  the  number 
depending  upon  the  intensity  of  the  light  desired. 

Artificial  light  is  to  be  preferred  as  it  is  much  more  uniform. 
The  following  table1  will  show  approximately  the  time  needed 
for  exposure: 

1  From  How  To  Make  Good  Pictures,  Eastman  Kodak  Co. 


Military  Topography  and  Photography  189 


g    J?  •§    *H  'S    « 

$  s  £ 

4x5,  or          7  inches  10  sec.  20  sec.  30  sec.  40  sec. 

smaller 

Regular  Velox  should  be  developed  to  the  proper  depth  in 
from  15  to  20  seconds;  Special  Velox,  30  seconds.  If  the  de- 
velopment is  faster  or  slower,  the  length  of  succeeding  ex- 
posures should  be*  accordingly  regulated.1 

DEVELOPING  AND  FIXING.  Prints  should  be  developed  in  the 
solution  recommended  by  their  manufacturers.  For  Velox  paper 
Nepera  Solution  should  be  used.  Should  the  photographer 
desire  to  mix  his  own  solution,  the  following  M.  Q.  Developer1 
should  be  used : 

(Dissolve  chemicals  in  the  order  named) 

Water  10  ozs.  300  c.      c. 

Elon  or  Metol  7  grains  y2  gram 

Hydrochinon  30  grains  2  grams 

Sulphite  of  Soda  (desiccated)       110  grains  7  grams 

Carbonate  of  Soda  (desiccated)     200  grains  13  grams 

Potassium  Bromide  (10%  Sol.)        40  drops  40  drops 

The  developing  is  carried  on  in  the  same  manner  as  for 
plates.  The  prints  are  put  in  the  developer  with  the  emulsion 
side  up,  several  at  a  time ;  the  developer  should  cover  the  emul- 
sion quickly  and  all  air  bubbles  removed  at  once  with  a  brush  of 
the  fingers.  The  trays  should  be  arranged  as  for  plate  de- 
veloping. The  temperature  of  the  developer  should  be  from 
65°  to  70°  F. 

Where  there  are  a  large  number  of  prints,  porcelain  lined 
dish  pans  can  be  used  to  advantage  for  both  developing  and  fix- 
ing. After  prints  are  developed,  they  should  be  rinsed  off  in 
pure  water  before  being  placed  in  the  fixing  bath.  In  order 
that  the  prints  may  be  thoroughly  fixed,  they  should  be  fre- 
quently changed  about  in  the  Hypo  in  order  to  bring  the  face  of 
all  prints  in  contact  with  the  Hypo.  The  following  Hypo 

1  From  How  To  Make  Good  Pictures,  Eastman  Kodak  Co. 


190  Military  Topography  and  Photography 

Formula1  is  recommended  where  prepared  powders  are  not  used : 
Water  64,  ozs.       .  1920  c.      c. 

Hyposulphite  of  Soda  (Crystal)  16  ozs.  480  grams 

When  thoroughly  dissolved,  add  the  following  hardening 

solution,  dissolving  the  chemicals  separately  and  in  the  order 
named : 

Water  5  ozs.  150  c.      c. 

Sulphite  of  Soda  (desiccated)  %  oz.  15  grams 

Acetic  Acid  No.  8  (25%)  3  ozs.  90  c.     c. 

Powdered  Alum  1  oz.  30  grams 

WASHING  AND  DRYING.  Prints  must  be  thoroughly  washed  in 
water  to  remove  all  traces  of  Hypo.  This  is  especially  difficult 
where  a  large  number  of  prints  must  be  washed  at  the  same 
time.  In  such  cases  it  is  best  to  have  two  washing  trays  or  tubs, 
changing  the  prints  from  one  tub  to  the  other  several  times, 
and  using  fresh  water  at  each  change. 

After  they  have  been  thoroughly  washed,  the  prints  are  ready 
to  be  dried.  They  should  be  placed  face  down  in  a  pile  on  a 
clean  piece  of  glass,  and  pressed  with  the  hands  to  remove  the 
surplus  water,  after  which  they  should  be  laid  out  singly,  and 
face  dowi;  on  cheese  cloth  stretchers — frames,  3  or  4  feet  square, 
on  which  cheese  cloth  has  been  stretched  and  fastened. 

MOUNTING.  If  it  is  desired  to  place  or  mount  the  print  on 
a  cardboard,  mounting  tissue  or  library  paste  is  applied  to 
the  back  of  the  print.  To  apply  the  mounting  tissue,  the  print 
is  placed"  face  down  on  a  clean  surface  and  a  piece  of  mounting 
tissue  of  the  same  size  is  fastened  to  its  back  by  touching  the 
tissue  at  several  places  with  the  point  of  a  hot  iron  (not  too 
hot)  ;  if  it  is  desired  to  trim  the  print  it  is  now  done,  using  a 
trimming  board — a  knife  or  scissors  should  never  be  used;  the 
print  is  then  placed  on  the  mount  with  the  tissue  next  to  it  at 
the  position  desired,  and  the  surface  of  the  print  pressed  with  a 
hot  iron — not  rubbed.  Prints  so  mounted  will  not  curl  even 
on  the  thinnest  mounts;  two  prints  may  even  be  placed  back  to 
back  with  the  mounting  tissue. 

Prints  for  military  purposes  will  seldom  be  mounted.  To 
remove  the  curl  formed  in  drying,  each  print  is  placed  face 
down  on  a  piece  of  clean  white  paper,  and  a  hot  iron  is  run  over 
its  surface. 


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Military  Topography  and  Photography  191 

INTENSIFICATION  AND  REDUCTION 

INTENSIFICATION.  When  negatives  are  underdeveloped,  they 
may  be  intensified  as  follows :  If  the  negative  has  been  allowed 
to  dry  it  should  be  soaked  in  clean  water  about  20  minutes. 
It  is  then  put  face  up  in  an  empty  tray  and  the  intensifier 
solution  poured  over  it.  The  intensifier  is  allowed  to  act  until 
the  negative  is  all  of  one  even  color ;  the  solution  is  then  poured 
off  and  the  negative  is  washed  in  four  or  five  changes  of  clean 
water  for  fifteen  minutes  and  put  out  to  dry. 

It  is  recommended  that  prepared  intensifies  be  used.  The 
following  intensifiers,1  however,  may  be  prepared : 

Sol.  A:    Bi-chloride  of  Mercury  (poison)    75  grains 

Water 5  ozs. 

Sol.  B :     Iodide  of  Potassium    112  grains 

Water 2'/2  ozs. 

Sol.  C:     Hyposulphite  of  Soda   150  grains 

Water     2y2  ozs. 

REDUCTION.  When  the  negative  is  overdeveloped  its  density 
may  be  reduced  in  the  following  manner:  If  the  negative  has 
been  allowed  to  dry,  it  is  soaked  in  clean  water  for  20  minutes, 
and  then  immersed  in  a  reducing  solution.  The  tray  is  gently 
rocked  back  and  forth  until  the  negative  has  been  reduced  to 
the  desired  density.  It  is  then  washed  in  running  water  for  10 
minutes,  or  in  four  changes  of  water.  The  following  reduction 
formula2  is  recommended: 

Water 6  ozs. 

Hyposulphite  of  Soda   %  oz. 

Ferri-Cyanide    of    Potassium    (saturated    solution), 

poison 20  drops 

PHOTOGRAPHIC  TERMS 

"HIGH  LIGHTS."  Those  portions  of  a  negative  which  are 
most  dense,  or  represent  the  bright  or  light  portions  of  the 
picture,  are  called  the  "High  Lights." 

"SHADOWS."  Those  portions  of  a  negative  which  are  most 
transparent,  or  represent  the  dark  portions  of  the  picture,  are 
called  the  "shadows." 

"THIN."  When  a  negative  has  little  density  in  the  "High 
Lights"  it  is  said  to  be  "Thin." 

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192  Military  Topography  and  Photography 

"DENSE."  When  a  negative  is  dark  all  over  it  is  said  to  be 
"Dense." 

"FLAT."  When  there  is  very  little  contrast  between  the 
"High  Lights"  and  the  "Shadows"  of  a  negative,  it  is  said  to 
be  "Flat." 

"FOGGED."  When  a  negative  shows  no  clear  or  transparent 
places,  even  in  the  "Shadows,"  it  is  said  to  be  "Fogged." 

DEFECTS  IN  NEGATIVES 

Too  "TniN."  Negatives  which  are  too  "thin,"  have  usually 
been  underdeveloped.  The  beginner  seeing  a  well-defined  image 
on  the  plate  generally  stops  the  developing  too  soon  and  a  thin 
negative  results  after  the  plate  has  been  "fixed."  With  ex- 
perience the  photographer  will  learn  when  the  plate  has  been 
developed  to  the  proper  point :  thin  negatives  can  be  intensified 
as  heretofore  explained. 

Too  "DENSE."  Negatives  which  are  too  "dense,"  have  usual- 
ly been  overdeveloped,  the  plate  having  been  left  in  the  de- 
veloper too  long.  Dense  negatives  can  be  reduced  as  heretofore 
explained. 

MUCH  CONTRAST  BUT  LITTLE  DETAIL  IN  SHADOWS.  If  the 
"high  lights"  of  a  negative  are  too  dense  and  the  "shadows"  too 
transparent,  the  plate  has  usually  been  underexposed.  Judging 
the  light  and  time  is  a  matter  of  experience.  The  beginner 
should  keep  a  record  of  exposures  in  order  to  overcome  errors  in 
the  judging  of  the  light  and  time. 

LITTLE  CONTRAST  BUT  MUCH  DETAIL  IN  SHADOWS.  If  the 
"high  lights"  of  a  negative  are  not  dense  enough  and  the 
"shadows"  not  transparent  enough,  the  plate  has  usually  been 
overexposed.  The  same  procedure  should  be  followed  as  in  the 
preceding  section. 

"FOGGED  NEGATIVES."  If  a  negative  is  "fogged"  all  over, 
including  the  margins  which  were  hid  by  the  plate  holder  during 
exposure  and  which  should  be  perfectly  clear,  the  trouble  has 
usually  .been  in  the  dark  room  or  in  the  dark  room  lamp.  If  in 
the  dark  room,  it  is  not  light  tight :  if  in  the  dark  room  lamp, 
the  lamp  either  emits  white  light  or  the  red  light  is  too  bright. 

LACK  OF  SHARPNESS.  If  the  picture  or  negative  lacks  sharp- 
ness and  definition,  either  the  camera  was  out  of  focus  or  was 


Military  Topography  and  Photography  193 

jarred  during  exposure.  The  remedy  in  the  first  case  is  the 
proper  'estimation  of  distance,  or  proper  focusing.  In  the  second 
case  the  camera  may  have  been  jarred  by  the  wind  during 
exposure,  or  when  the  bulb  was  pressed.  A  timed  exposure 
can  never  be  made  while  holding  the  camera  in  the  hands. 

SPREADING  OF  HIGH  LIGHTS.  In  photographing  objects  in 
which  there  is  a  great  contrast  of  light  and  shade,  such  as 
interiors  where  a  window  is  included  in  the  view,  the  intensity 
of  the  light  from  the  bright  portions  will  illuminate  the  images 
of  the  dark  portions  and  thus  blot  out  the  detail  from  the  black 
portions.  The  remedy  here  is  to  use  a  non-halation  plate  which 
overcomes  this  illumination,  or  "halation"  effect. 

BLACK  STREAKS  OR  BLOTCHES.  A  negative  which  has  black 
streaks  or  blotches  on  it,  has  been  light  struck  either  before 
or  after  exposure;  that  is,  direct  light  has  struck  the  plate. 
This  may  result  from  a  vast-  number  of  causes — leaky  plate 
holders,  non-light-tight  dark  room,  dark-room  lamp  emitting 
white  light,  mistakes  by  the  photographer,  etc. 

FROSTY  APPEARANCE.  Should  the  negative  a  few  days  after 
drying,  show  a  white  appearance  or  deposit  on  the  film,  it  has 
not  been  thoroughly  washed,  this  appearance  being  caused  by 
the  Hypo  still  remaining  in  the  film.  The  negative  must  be 
thoroughly  washed :  Hypo  remaining  in  the  negative  will  affect 
the  prints  made  from  it,  even  before  it  shows  on  the  negative 
itself. 

FINGER  PRINTS.  Finger  marks  on  the  negative  are  caused 
by  touching  the  emulsion  side  of  a  dry  plate,  or  allowing  the 
plain  side  of  a  dry  plate  that  has  been  touched  with  the  fingers 
to  come  in  contact  with  the  emulsion  side  of  another  dry  plate, 
before  exposure.  Dry  plates  should  always  be  picked  up  by 
the  edges,  and  when  packed  placed  emulsion  sides  together. 
Finger  marks  cannot  be  removed  from  the  negative.  In  re- 
moving plates  from  plate  holders  after  exposure,  they  should 
be  similarly  handled  by  their  edges  only. 

STAINS.  Yellow  or  brown  stains  are  generally  caused  by 
using  a  developer  that  has  been  allowed  to  spoil  from  age  or 
uncleanliness.  They  are  also  caused  by  using  water  containing 
iron,  or  a  rusty  developer,  fixing,  or  washing  tray.  The 


194  Military  Topography  and  Photography 

"whites"  (or  "high  lights")  of  prints  will  have  a  yellowish  tone 
or  small  red  spots  should  water  containing  iron  be  used.  This 
may  be  prevented  by  filtering  the  water  through  several  thick- 
nesses of  flannel,  or  one  of  canton  flannel,  before  using  it  in 
solutions.  Iron  rust  stains  can  be  removed  by  soaking  the 
negative  for  a  few  moments  in  diluted  Sulphuric  Acid  (1:20). 

PIN  HOLES.  Pin  holes  are  caused  by  particles  of  dust  adher- 
ing to  the  emulsion  surface  of  the  dry  plate  during  exposure. 
Before  dry  plates  are  loaded,  this  emulsion  surface  should  be 
lightly  brushed  with  a  soft  camel  hair  brush  to  remove  all 
traces  of  dust. 

TRANSPARENT  SPOTS.  Transparent  spots  are  caused  by 
bubbles  forming  and  remaining  on  the  dry  plate  during  the 
developing.  As  soon  as  a  bubble  forms  on  the  plate  during 
the  Developing,  it  should  be  immediately  removed  by  a  light 
brush  of  the  finger. 

OPAQUE  SPOTS.  Opaque  spots  on  the  negative  are  caused  by 
dust  adhering  to  the  plate  while  immersed  in  the  developer,  or 
from  dirt  in  the  developer,  or  in  the  fixing  solution,  or  in  the 
washing  water.  The  plate  before  being  immersed  into  the  de- 
veloper, should  have  its  emulsion  surface  lightly  brushed  with 
a  soft  camel  hair  brush  to  remove  any  trace  of  dust  that  may 
be  on  it.  The  chemicals  used  and  their  solutions  should  not 
only  be  pure,  but  the  dark  room  should  also  be  scrupulously 
clean  so  that  no  loose  dirt  or  dust  will  fall  in  the  trays  during 
the  developing,  fixing,  and  washing  processes. 

TRANSPARENT  LINE^.  Transparent  lines  on  the  negative  are 
caused  by  brushing  the  emulsion  surface  of  the  dry  plate  too 
hard,  or  by  using  a  stiff  brush.  The  brush  should  be  a  soft 
camel  hair  brush,  and  the  emulsion  surface  should  be  brushed 
very  lightly  and  carefully.  This  brush  should  be  used  for  no 
other  purposes. 

OPAQUE  LINES.  Opaque  lines  are  generally  caused  by  chemi- 
cals getting  on  the  hairs  of  the  brush  used  to  dust  the  emulsion 
surface  of  the  dry  plate.  This  brush  should  never  be  laid  down 
on  the  work  bench,  but  should  have  a  special  place  on  the  shelf 
above  assigned  to  it  where  there  is  no  probability  of  its  coming 
in  contact  with  chemicals,  either  dry  or  in  solution. 


Military  Topography  and  Photography  195 

MOTTLED  APPEARANCE.  The  negative  will  present  a  mottled 
appearance  if  the  developer  does  not  uniformly  and  quickly 
cover  the  whole  plate,  and  also  if  the  developer  is  not  kept  in 
constant  motion  over  the  emulsion  surface  by  the  tray  being 
slowly  and  continuously  rocked  back  and  forth  during  the 
developing. 

DEFECTS  IN  PRINTS1 

Having  given  a  good  negative  from  which  the  prints  are 
made,  the  following  defects  may  result: 

Prints  are  too  black:  Ove^exposure ;  overdevelopment;  in- 
sufficient Bromide  of  Potassium;  wrong  grade  of  paper  used. 

Prints  too  light:  Underexposure;  underdevelopment ;  wrong 
grade  of  paper. 

Not  enough,  or  too  much,  contrast :  Usually  happens  where 
whites  develop  too  quickly  for  shadows  to  develop  proper  defi- 
nition, or  shadows  develop  too  quickly  for  whites.  Expose 
shadows  longer  or  shorter  than  high  lights  to  the  light. 

Grayish  whites  throughout  entire  print:  Chemical  or  light 
fog;  insufficient  Bromide  of  Potassium. 

Grayish  mottled  or  granulated  appearance  of  edges  or  of 
entire  print:  Underexposure;  forced-development;  old  paper; 
paper  kept  in  damp  place;  moisture;  chemical  fumes. 

Brown  or  red  stains:  Old  or  oxidized  developer;  developer 
too  warm;  imperfect  fixing;  fixing  bath  lacks  sufficient  acid 
and  prints  are  not  kept  moving  in  it  to  allow  even  fixing. 

Round  white  or  black  spots:  Air  l:clls  on  surface  of  emul- 
sion ;  develop  prints  face  up ;  remove  air-bells  when  formed. 

White  deposit  all  over  surface  of  prints :  Milky  Hypo  bath ; 
insufficient  acetic  acid  in  Hypo  bath. 

Picture  good,  but  surface  covered  with  black  marks :  Abra- 
sion marks.  For  glossy  surface  paper  use  N.  A.  (Non-abra- 
sive) Velox  Liquid  Developer. 

Blisters :  Prints  creased  or  broken  while  washing ;  water 
from  tap  falling  directly  on  prints ;  too  strong  acetic  acid  used 
in  hardener ;  too  great  difference  between  temperature  of  fixing 
solution  and  wash  water;  fixing  bath  lacks  sufficient  hardener. 
Never  use  a  plain  Hypo  fixing  bath  without  a  hardener. 

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196  Military  Topography  and  Photography 

MILITARY  PHOTOGRAPHY 

POSITION  AND  OUTPOST  VIEWS.  There  will  often  not  be 
sufficient  time  to  make  sketches  of  position  and  outpost  areas. 
In  such  cases  photographs  of  the  areas,  until  the  proper 
sketching  can  be  done,  will  be  invaluable.  Photographs  of 
sketched  areas  will  also  be  valuable  supplements  to  the  maps 
thereof.  In  photographing  an  area,  views  should  be  taken 
from  several  positions  if  possible.  Where  a  photograph  of  a 
wide  front  Js  desired,  a  panoramic  camera  can  be  used  to  great 
advantage. 

PLACE  AND  RECONNAISSANCE  VIEWS.  Place  views  will  usually 
be  taken  while  out  on  reconnaissance  work,  and  may  or  may 
not  be  supplemental  to  a  sketch  of  the  terrain  included  by  the 
photograph.  The  scout  will  find  a  Pocket  Kodak  or  No.  0 
Graflex,  or  other  similar  cameras,  best  adapted  to  reconnais- 
sance work.  These  cameras  are  small  and  can  be  easily  carried 
on  the  person  and  will  not  incumber  his  movements. 

TOPO-PHOTOGRAPHY.  This  subject  has  been  fully  treated 
under  Photo-Topographic  Operations  in  the  chapter  on  Mili- 
tary Topography. 

AERO-PHOTOGRAPHY.  Whenever  a  photograph  of  the  terrain 
is  taken  with  a  single  camera,  as  a  Graflex,  from  an  aeroplane, 
the  camera  will  usually  be  held  at  an  inclined  angle,  and  an 
oblique  view  of  the  plane  of  the  objective  (in  this  case,  the 
ground)  will  be  obtained.  This  view  will  be  distorted,  for  the 
ground  glass  of  the  camera  will  not  be  parallel  with  the  plane 
of  the  objective.  Thus,  in  Fig.  77,  a  and  a1  are  equal  angles, 
and  the  intercepts  niim2  and  m2m3  on  the  ground  glass  are  also 
equal  to  each  other,  but  the  intercept  MiM2  is  much  less  than 
the  intercept  M2M3  on  the  ground.  Now  MiM2  occupies  as 
much  space  (mim2)  on  the  negative,  or  ground  glass,  as  M2M3, 
(m2m3)  does,  and  thus  m2m3  is  distorted  with  respect  to  mim2, 
as  well  as  the  whole  outer  field  with  respect  to  the  inner  field 
of  the  view.  If  the  ground  glass  were  parallel  with  the  plane 
of  the  objective,  as  the  line  rp2  in  Fig.  77,  the  view  would  not 
be  distorted,  but  it  is  impossible  to  adjust  the  ground  glass  for 
each  exposure  in  aero  work. 


Military  Topography  and  Photography 


197 


FIG.  77 

RECTIFICATION  OF  DISTORTION.  It  is  obvious  that  the  dis 
torted  view  thus  obtained  should  be  rectified  into  a  true  view. 
If  a  distorted  negative  be  placed  in  front  of  a  camera  so  as  to 
make  the  same  angle  with  the  optical  axis  of  the  camera  as  the 
optical  axis  made  with  the  plane  of  the  objective  when  the 
negative  was  taken,  and  a  photograph  of  that  negative  token, 
the  resulting  positive  image  on  the  dry  plate  will  be  a  corrected 
view.  Such  a  simple  procedure  in  the  rectification  of  distorted 
negatives  cannot  be  followed,  for  it  will  not  be  known  just  at 
what  angle  the  optical  axis  made  with  the  plane  of  the  objec- 
tive, and  the  true  scale  of  the  image  cannot  be  thus  reproduced, 
or  rather  produced. 

If  in  Fig.  77,  the  camera  lens  O  be  imagined  wide  angled 
enough  to  permit  a  vertical  ray  from  M  to  pass  through  to 
intersect  the  ground  glass,  m3mi  produced,  at  point  m,  then  the 
hypothetical  image  point  m  is  the  only  point  on  the  negative 
where  the  view  is  not  distorted.  From  zero  at  point  m  (M) 
this  distortion  increases  directly  in  magnitude  toward  m3  (M3), 
so  that  even  image  nil  on  the  near  edge  of  the  view  will  be  out 
of  scale  (distorted)  where  the  altitude  of  the  camera  is  con- 
sidered the  distance  to  the  plane  of  the  objective.  When  an 
object  is  twice  the  focal  distance  from  one  side  of  a  lens,  the 
image  of  it  is  twice  the  focal  distance  on  the  other  side  of  the 
lens  and  is  of  the  same  magnitude.  Therefore,  to  take  a  photo- 


198 


Military  Topography  and  Photography 


graph  of  a  negative  so  that  it  will  be  produced  to  scale,  the 
hypothetical  point  m  on  the  negative  and  its  hypothetical  image 
point  m'  on  the  positive  must  be  on  opposite  sides  of  the  lens 
and  both  twice  the  focal  distance  away.  Then  m  and  m'  will 
be  of  the  same  magnitude.  The  angles  at  which  the  negative 
and  positive  must  be  placed  with  respect  to  each  other  is  shown 
in  the  diagram  in  Fig.  78. 

This  diagram  shows  the  principle  of  the  Scheimpflug  Per- 
spectograph.     In  this  diagram  the  parallel  lines  are  a  focal 


distance  apart:  the  lines  KA  and  KB  form  the  angles  a  and  S 
with  the  central  line,  designated  "Plane  of  Objective."  a  and 
P  are  found  from  the  following  formula: 

mv  sin  a  =  f  =  m1r  sin  j$ 

in  which  mv  and  'mV  (or,  mr)  can  be  found  from  Fig.  77. 
m11m13  is  the  corrected  positive  of  negative  mim3,  and  it  will  be 
noticed  that  m1m1i  :m1im13 :  rMM^  :MiM8,  so  that  the  intercepts 
on  the  corrected  positive  are  proportional  to  the  corresponding 
intercepts  on  the  ground  throughout. 

THE  SCHEIMPFLUG  CAMERA..    A  vertical  photograph  of  the 
terrain  taken  from  a  point  above,  such  as  from  a  balloon,  will 


Military  Topography  and  Photography  199 

include  only  a  small  field  even  when  taken  from  a  comparatively 
high  altitude  and  a  wide  angle  lens  is  used.  Thus,  with  a  60° 
angle  lens  at  an  altitude  of  one  mile,  the  field  will  be  a  circle 
whose  diameter  is  only  about  6,000  feet.  Most  aero-photo- 
graphs, however,  are  taken  at  altitudes  much  less  than  one  mile 
and  the  field  is  proportionally  smaller.  For  successful  aero- 
photography,  therefore,  a  special  camera  with  large  field  must 
be  used.  For  such  work,  Captain  Theodore  Scheimpflug  of  the 
Austrian  Army,  has  devised  a  compound  camera  which  has  met 
with  the  greatest  success,  and  the  general  principles  of  it  will 
be  discussed. 

The  Scheimpflug  Camera  consists  of  one  central  vertical 
camera  surrounded -by  seven  other  cameras  inclined  at  an  angle 
of  45°  to  it.  The  shutters  of  all  these  cameras  work  simultane- 
ously and  are  controlled  by  one  release.  The  combined  expo- 
sures include  an  angle  of  140°  and  cover  a  field  over  25  times 
as  large  as  a  single  camera.  Fig.  79  (a)  shows  the  general" 
principle  of  the  Scheimpflug  Camera,  while  Fig.  79  (b)  shows 
its  photographic  field.  Since  there  are  seven  inclined  cameras 
evenly  spaced  around  the  central  vertical  one,  no  two  cameras 
are  directly  opposite  each  other.  In  Fig.  79  (a)  however,  the 
liberty  is  taken  of  representing  two  inclined  cameras  in  the 
same  vertical  plane  as  the  central  camera,  so  as  to  show  more 
clearly  the  vertical  intercepts  on  the  ground. 

The  intercept  or  field  of  the  central  camera  is  a  circle  deter- 
mined by  the  intersection  of  a  vertical  cone  with  the  ground: 
the  apex  of  this  cone  is  the  lens  of  this  camera  and  angle  of  the 
apex  of  this  cone  is  equal  to  the  angle  of  the  lens.  The  cones 
of  light  from  the  inclined  cameras  intercept  the  ground  at  an 
angle  of  45°.  Fig.  79  (b)  shows  the  photographic  field  of  the 
compound  camera.  The  intersection  of  fields  of  the  inclined 
cameras  are  shown  only  within  the  working  limits  of  the  com- 
bined cameras.  It  will  be  noticed  that  the  field  of  each  camera 
must  overlap  the  adjacent  fields  so  that  the  entire  field  is 
covered. 

The  combined  photographic  field  is  thus  made  up  of  one 
central  vertical  view  and  seven  surrounding  oblique  views.  The 
distortion  of  the  oblique  views  is  corrected  by  means  of  the 


PHOTOGRAPHIC  FIELD 
FlO.  79 


? 


go 

I  a 


IE 


202 


Military  Topography  and  Photography 


*VlEW    OF    SCHEIMPFLUG    CAMERA    FROM    BELOW 

perspectograph,  the  same  as  explained  for  an  oblique  view  from 
a  single  camera,  and  these  corrected  views  are  assembled  photo- 
graphical  to  the  central  view  with  microscopic  precision  by 
means  of  a  special  camera. 

*0ourtesy  of  Scheimpflug  Institute 


u>    o 

if 

I  a 

5 1 


AEREO-RECONNAISSANCE   VIEW  DISTORTED 
Courtesy  of  Scheimpflug  Institute 


AEREO-RECONXAISSANCE   VIEW   CORRECTED 

Courtesy  of  Scheimpflug  Institute 


CHAPTER  V 

SPECIAL  PROBLEMS 

CHAINING 

In  chaining  the  distance  between  two  points  four  things  must 
be  kept  in  mind:  (1)  the  straight  distance  between  the  two 
points  is  to  be  obtained;  (2)  the  horizontal  distance  between 
the  two  points  is  to  be  obtained;  (3)  the  point  where  the  end' 
of  the  chain  or  tape  comes  each  time  must  be  marked;  and 
(4)  the  chain  or  tape  must  be  stretched  straight  and  tight 
each  time. 

READING  THE  CHAIN  OR  TAPE.  The  Gunter's  Chain  is 
66  feet  (4  rods)  long  and  has  100  links.  Each  link  is  therefore 
1/100  of  a  chain,  and  the  number  of  links  are  written  decimally. 
Thus,  a  distance  containing  five  chains  and  seven  links  would 
be  written,  5.07  chains.  The  Surveyor's  Chain  is  100  feet  long 
and  has  100  links  each  1  foot  long.  Chains  usually  have  every 
tenth  link  numbered  with  a  tag  from  the  ends  towards  the 
center;  as,  0-10-20-30-40-50-40-30-20-10-0.  In  reading,  two 
very  easy  errors  must  be  avoided:  60  feet,  etc.,  must  not  be 
read  as  40  feet,  etc.,  and  42  feet,  etc.,  for  38  feet,  etc.  The 
latter  can  be  avoided  by  reading  the  tags  on  each  side. 

The  tape  is  usually  100  feet  long,  and  is  divided  into  feet  with 
the  last  foot  on  each  end  divided  into  tenths  and  hundredths  of 
a  foot.  Some  tapes  are  numbered  to  correspond  to  chains,  and 
such  tapes  possess  all  the  difficulties  of  a  chain  in  reading.  Most 
chains,  however,  have  their  divisions  numbered  consecutively 
from  "0"  to  "100."  When  the  distance  between  two  points  is 
less  than  the  length  of  the  tape,  the  tape  is  stretched  from  one 
stake  to  beyond  the  second  stake;  the  head  chainman  goes  to 
the  second  point,  puts  the  next  greater  foot  division  opposite  the 
second  point  while  the  rear  chainman  pulls  the  tape  straight  and 
tight,  and  notes  the  subdivision  in  tenths  and  hundredths  which 
is  opposite  the  first  point.  The  head  chainman  announces  the 
number  of  the  division  opposite  the  second  point,  and  the  dis- 
tance is  read  by  the  rear  chainman  mentally  subtracting  from 


Military  Topography  and  Photography  207 

that  number  1  foot  and  adding  the  fraction  of  a  foot  from  the 
first  point  to  the  "1-ft."  division. 

CHAINING.  A  chaining  party  will  usually  consist  of  a  front 
and  rear  chainman,  designated  as  No.  1  and  No.  2,  respectively, 
equipped  with  a  100  foot  tape,  two  markers  and  11  pins.  Pro- 
cedure : 

When  the  two  points  are  intervisible  and  directly  accessible: 
Both  points  are  marked  with  rods  or  markers  so  that  they  can 
be  easily  seen.  The  tape  having  been  stretched  out  on  the 
ground  with  all  bends  and  kinks  removed,  No.  1  who  has  the 
11  pins  sticks  one  at  the  initial  point;  he  then  picks  up  the 
100  foot  end  of  the  tape  and  walks  directly  towards  the  far 
point ;  when  the  "0"  end  of  the  tape  nears  the  initial  point,  No.  2 
calls  out  "Chain!"  at  which  No.  1  halts,  faces  No.  2,  and  pulls 
the  tape  fairly  taut ;  at  this  instant  No.  2  clasps  the  "0"  end  of 
the  tape,  kneels  directly  in  rear  of  the  initial  point,  with  the  left 
foot  in  such  a  position  that  it  forms  a  prop  for  the  hands  when 
the  "0"  division  of  the  tape  is  opposite  the  far  end  of  the  pin; 
No.  2  Ihcn  motions  No.  1  to  the  right  or  left  in  alignment  with 
the  far  point,  after  which  No.  1  pulls  the  tape  straight  and 
tight,  taking  the  kneeling  position  facing  No.  2 ;  No.  2,  observ- 
ing that  the  "0"  division  is  opposite  its  proper  place,  calls  out, 
"Stick!";  No.  1  then  sticks  a  pin  with  its  far  side  opposite  the 
100  foot  division  and  calls  out,  "Stuck!";  No.  2  then  lets  loose 
of  the  tape  and  pulls  up  the  pin  at  the  initial  point  and  follows 
the  end  of  the  tape  which  is  being  pulled  forward  by  No.  1.  As 
the  second  pin  is  approached,  the  same  procedure  is  followed, 
and  so  on,  throughout  the  distance.  When  No.  1  has  stuck  his 
last  pin,  he  calls  out  "Pins!,"  and  No.  2  goes  forward  to  him  with 
the  pins  which  he  has  pulled  up,  and  as  a  check  counts  them  for 
there  should  be  10  (one  is  always  left  in  the  ground).  It  is 
evident  that  each  time  No.  1  calls  for  pins  he  will  have  gone 
1000  feet.  If,  therefore,  at  the  far  point,  No.  2  has  six  pins  in 
his  hands  No.  1  has  called  for  "pins"  three  times,  and  from  the 
last  pin  stuck  to  the  far  point  is  43  feet,  the  distance  measured 
between  the  two  points  is  3643  feet. 

When  the  two  points  are  not  intervisible,  but  directly  acces- 
sible: This  condition  will  occur  where  a  ridge  or  other  high 
ground  is  between  the  two  points.  In  such  a  case  the  two  points 


c.  /> 

FIG.  85 


FIG.  86 


O       F 


FIG.  87 


FIG.  88 


FIG.  89 


Military  Topography  and  Photography  209 

are  first  marked  with  visible  rod,  after  which  the  two  chainmen 
proceed  to  the  intervening  ridge  or  hill,  No.  1  placing  himself 
on  the  far  side  of  the  hill  so  that  he  can  just  see  the  near  point, 
while  No.  2  places  himself  on  the  near  side  of  the  ridge  just  so 
he  can  see  the  far  point.  No.  1  then  aligns  No.  2  to  the  right 
or  left  on  the  near  point,  after  which  No.  2  aligns  No.  1  to  the 
right  or  left  on  the  far  point.  They  then  alternately  align  each 
other  until  they  are  both  on  the  line,  as  shown  in  Fig.  85. 
They  mark  their  positions  with  rods,  and  then  commence  chain- 
ing, using  these  intermediate  rods  first  for  aligning,  and  after 
the  high  ground  is  reached  the  visible  rod  marking  the  far 
point. 

'When  the  points  are  intervisible,  but  not  directly  accessible: 
This  condition  will  occur  where  a  lake  or  swamp  intervenes.  In 
such  a  case,  the  chainmen,  one  on  the  near  side  of  the  lake  and 
the  other  on  the  far,  align  themselves  on  the  straight  line  join- 
ing the  two  points,  as  in  the  preceding  case,  and  mark  their 
position  with  visible  rods.  The  distance  from  the  first  point 
to  the  near  rod  is  first  chained;  a  line  at  right  angle  to  this 
line  is  then  chained  far  enough  to  clear  the  lake ;  then  a  line  at 
right  angle  to  this  last  line  is  then  chained,  in  the  original 
direction,  far  enough  to  pass  the  point  on  the  far  point  of  the 
lake;  a  line  is  then  chained  at  right  angle  to  the  last  line  until 
it  intersects  the  straight  line  joining  the  two  points  whose 
distance  apart  is  being  measured.  Fig.  86  shows  this  method 
graphically.  From  the  figure  it  is  evident  that  if  A  and  B  are 
the  two  points,  their  distance  apart  is  equal  to  AC+EF+GB. 
Local  conditions  might  make  an  equilateral  or  right  triangle 
offset  better  than  a  rectangle  offset,  as  shown  in  Figs.  87  and 
88,  the  side  CF  being  obtained  by  calculation. 

When  the  two  points  are  neither  intervisible  nor  directly 
accessible:  This  condition  will  occur  where  a  woods  or  under- 
brush intervenes.  When  the  intervening  vegetation  is  not  too 
large,  select  a  point  C  which  is  intervisible  and  directly  ac- 
cessible to  both  A  and  B,  Fig.  89.  Chain  the  distances  AC  and 
CB;  at  a  convenient  known  point  D  on  the  line  AC,  measure  off 

ADXCB 

a  line  DE  parallel  to  CB,  and  equal  to  -        .     If  the  fore- 

AC 


210  Military  Topography  and  Photography 

going  has  been  carefully  done,  the  line  AE  produced  will  mark 
the  line  to  be  cleared  through  the  woods  in  order  to  make  A  and 
B  intervisible  and  accessible.  Should  the  woods  be  of  any  extent 
it  will  be  far  more  economical  to  run  a  transit  traverse  to 
determine  their  distance  apart. 

CARE  OF  STEEL  TAPE  AND  CHAIN.  The  steel  tape  is  easily 
broken  in  two  if  pulled  when  kinked,  and  before  pulling  it  along 
on  the  ground  or  stretching  it  to  make  straight  and  tight,  the 
front  chainmen  should  look  along  the  length  of  the  chain  each 
time  to  see  that  there  are  neither  kinks  nor  bends.  Both  tape 
and  chain  easily  rust,  so  that  each  time  before  putting  the  same 
away  they  should  be  wiped  dry ;  if  stored  away  they  should 
have  a  good  coating  of  cosmoline  or  other  heavy  grease.  In- 
stead of  carrying  a  steel  tape  on  a  closely  wound  reel,  it  is 
better  to  carry  it  looped.  This  is  done  as  follows:  The  near 
end  of  the  tape  is  clasped  in  the  right  hand  palm  up,  and  the 
right  hand  is  swung  to  the  rear  pulling  the  chain  with  it;  the 
chain  is  then  clasped  by  the  left  hand  palm  down  at  the  first 
five  foot  division  and  placed  in  the  palm  of  the  right  hand 
without  allowing  the  tape  to  turn;  each  five  foot  division  is 
similarly  placed  over  the  palm  of  the  right  hand,  and  the  tape 
is  thus  gathered  in  concentric  folds  which  can  be  crossed  and 
tied  at  the  center.  The  chain  is  first  doubled  in  the  middle,  after 
which  each  two  links  are  folded  together  across  the  two  preced- 
ing links  so  that  the  whole  chain  when  folded  presents  the 
appearance  of  an  hour  glass.  To  undo  a  tape  or  chain  so 
done  up,  reverse  the  above  processes. 

TEUEMETRIC  MEASUREMENT  OF  DISTANCES 
THEORY  OF  THE  STADIA.  In  the  measurement  of  distance 
with  the  stadia,  the  intercept  which  two  horizontal  stadia  wires 
in  the  telescopic  field  makes  on  a  graduated  rod  held  on  the 
distant  point  is  observed.  This  intercept  is  directly  propor- 
tional to  the  distance.  Fig.  90  shows  the  stadia  rod  as  seen 
through  the  telescope. 

From  the  theory  of  lenses,  it  will  be  noticed  from  Fig.  91  that 
all  the  rays  of  light,  such  as  A  and  B,  pass  through  the  common 
point  0,  one  of  the  foci  of  the  lens,  and  after  passing  through  the 
lens  become  parallel  rays  aa1,  bb1  and  so  on.  In  the  similar 


Military  Topography  aiid  Photography 


211 


FIG.  90 


triangle,  Orfb1  and  OAB,  fiSn^b1:  AB,  but  ab  =  albl;  there- 

fXAB 

'.    The  reticle  containing  the  horizontal  cross 


fore  S  — 


ab 


wires  is  placed  at  the  focal  distance  of  the  lens  in  rear  of  it, 


-E-:-rr-vi-> 


p 


FIG.  91 

and  the  distance  apart  of  the  two  horizontal  wires  a  and  b  is 
usually    fixed    and    of    such    amount    that    the    focal    distance 

divided  by  ab  (usually  designated  as  — )  equals  100.     In  such 

i 

cases  S  ==  100  X  AB.     S-  is  the  distance  from  0  to  the  stadia 
rod.  so  that  if  the  distance  from  the  center  of  the  telescope  is 


212 


Military  Topography  and  Photography 


desired,  as  in  more  accurate  work,  the  constant  f+c  (focal 
length  of  lens  plus  distance  from  center  of  transit  to  lens) 
must  be  added  to  each  stadia  reading  made  on  the  stadia  rod. 
This  "c  +  / "  is  the  same  for  all  distances. 

INCLINED  READINGS.  If  in  Fig.  92,  the  stadia  rod  AB  were 
held  perpendicular  to  the  line  of  collimation,  then  the  stadia 
reading  obtained  would  be  the  slope  distance ;  but  the  stadia 


FIG.  92 

rod  is  always  held  vertical,  and  the  actual  intercept  A^1  is 
larger  than  AB.  Let  a  be  the  vertical  angle  of  the  telescope. 
In  Fig.  92,  the  angle  at  A  can  be  taken  as  a  right  angle,  and 
the  .angle  A^A  equals  angle  a,  from  which  100AB  = 
lOOA^cosa  =  the  slope  distance  Om.  The  slope  distance, 
OM,  times  cos  a  equals  the  horizontal  distance  S,  therefore : 
H  =  (c  +  f)  cos  a  +  (lOOA'B1  cos  a)  cos  a,  or 

=  (c  +  f)  cos  a  +  S  cos2  «/ 

To  get  the  difference  in  elevation  it  is  only  necessary  to  mul- 
tiply the  slope  distance  (A^B1  cos  a)  by  the  sine  of  the  vertical 
angle,  or: 


Military  Topography  and  Photography  213 

Em  =  (c  +  f)  sin  a  +  (A1!*1  cos  a)  sin  a,  and  since, 
cos  a  sin  a  =  l/2  sin  2«,  and  S  =  lOOA'B1 ; 

Em  ==  (c  +  f )  sin  a  +  S  1/2  sni  2a. 

The  values  for  H  and  Em  for  vertical  angles  up  to  30°  for 
stadia  reading  of  100  are  given  in  Table  IV. 

f 

The  value  —  is  seldom  exactly  100,  but  slightly  less  or  more, 

i 

so  it  is  necessary  to  multiply  the  stadia  reading  read  by  a  small 
factor,  called  the  stadia  constant.  The  method  of  obtaining 
this  standia  constant  is  explained  later  on. 

VERNIERS 

A  vernier(is  an  auxiliary  scale  by  which  in  conjunction  with 
the  main  scale  the  latter  can  be  read  more  closely  than  can 
be  shown  by  actual  subdivision. 

\     '.     '     '  .  ' 


T 1 

r 


FIG.  93  —  DIRECT  VERNIER 


DIRECT  VERNIERS.  In  Fig.  93  the  upper  scale  is  the  vernier 
scale,  while  the  lower  scale  is  part  of  the  main  scale  to  which  the 
former  belongs.  It  will  be  noticed  that  ten  divisions  are  equal 
to  just  nine  of  the  divisions  on  the  main  scale,  from  which  it 
can  be  easily  seen  that  each  division  on  the  vernier  is  just  one- 
tenth  smaller  than  the  division  on  the  main  scale.  If,  therefore, 
the  vernier  scale  be  moved  just  one-tenth  of  a  division  to  the 
right,  the  first  line  of  the  vernier,  i.  e.,  the  first  line  to  the  right 
of  the  "0,"  will  coincide  with  a  line  on  the  main  scale  ;  if  moved 
two-tenths  of  a  division  to  the  right  the  second  line  on  the 
vernier  will  coincide  with  a  line  on  the  main  scale,  and  so  on. 
It  will  be  seen  that,  except  when  the  "0"  line  coincides,  only 
one  line  on  the  vernier  can  coincide  with  a  line  on  the  main  scale 
at  the  same  time. 

Where  a  vernier  is  used  in  conjunction  with  lineal  scale,  such 
as  a  level  rod,  the  main  scale  will  be  subdivided  decimally  as, 
feet,  tenths  of  a  foot,  and  hundredths  of  a  foot,  while  the  vernier 


214  Military  Topography  and  Photography 

will  read  one-tenth  smaller  than  the  smallest  division  of  the 
main  scale,  or  thousandths  of  a  foot. 

To  read  a  vernier:  In  Fig.  93  assume  that  the  fifth  line  of 
the  vernier  coincides,  then  the  "0"  of  the  vernier  will  be  between 
the  "4"  and  "V  Look  along  the  main  scale  in  the  direction 
in  which  it  is  numbered  to  the  first  division  immediately  opposite 
(and  to  the  left  of)  the  "0"  of  the  vernier:  this  will  give  the 
reading  to  the  lowest  subdivision  on  the  main  scale:  then  look 
along  the  vernier  in  the  direction  in  which  it  is  numbered  to  the 
line  which  coincides,  which  will  give  the  next  lower  digit  of  the 
reading.  Under  the  above  supposition  the  reading  in  Fig.  93 
would  be  4.05,  but  as  actually  shown  is  4.00. 

RETROGRADE,  VERNIERS.  In  Fig.  94  it  will  be  noticed  that 
ten  divisions  on  the  vernier  are  equal  to  eleven  divisions  on  the 

'*  5 

i      i 


f    f 


i      i      i      <      I       »      t 

I      I      I      I      I      I      I 

|>:^?$>^> 


FIG.  94  —  RETROGRADE 


main  scale.  Each  division  on  the  vernier  is  therefore  one-tenth 
larger  than  the  smallest  division  of  the  main  scale.  If,  there- 
fore, the  vernier  be  moved  one-tenth  of  a  division  to  the  right, 
the  first  division  line  to  the  left  on  the  vernier  will  coincide,  and 
so  on.  For  this  reason  the  vernier  must  be  numbered  in  the 
direction  opposite  to  which  the  main  scale  is  numbered.  Such 
verniers  are  called  retrograde  verniers. 

To  read  a  retrograde  vernier:  Look  along  the  main  scale  to 
the  first  division  immediately  opposite  (and  to  the  left  of)  the 
"0"  of  the  vernier,  which  will  give  the  lowest  reading  on  the  main 
scale  :  then  look  back  along  the  vernier,  in  the  direction  in  which 
it  is  numbered  to  the  line  that  coincides,  which  will  give  the 
vernier  reading. 

Retrograde  verniers  are  little  used,  being  employed  in  level 
rods  numbered  from  the  center  downwards;  for  declination 
correction  scales  in  the  compasses  of  some  transits  where  an 
economy  of  space  is  desired;  and  in  a  few  other  special  cases. 


Military  Topography  and  Photography 


215 


LEAST  COUNT  OF  THE  VERNIER.  The  Value  obtained  by 
dividing  the  smallest  division  of  the  main  scale  by  the  number 
of  divisions  on  the  vernier  is  called  the  least  count  of  the 
vernier.  This  is  merely  the  reading  of  the  vernier  as  explained 
in  the  preceding  paragraphs,  and  may  be  denoted  by  1  -f-  n, 
where  1  equals  the  length  of  the  smallest  division  on  the  main 
scale  and  n,  the  number  of  divisions  on  the  vernier.  If,  in  case 
of  a  level  rod,  1  equals  one-hundredth  of  a  foot,  and  n  equals  10, 
then  the  least  count  on  the  vernier  will  be  one-thousandth  of  a 
foot.  In  a  transit  reading  directly  to  30',  and  n  equals  30,  the 
least  count  on  the  vernier  jvvill  be  V.  In  all  direct  verniers, 
where  n  is  the  number  of  divisions  on  the  vernier,  n  —  1  will 
be  the  number  of  divisions  on  the  main  scale  which  are  equal 
to  n  divisions  of  the  vernier. 

DOUBLE  VERNIERS.  In  case  of  the  horizontal  scale  of  a 
transit,  which  is  used  to  measure  angles  in  either  direction,  two 
direct  verniers  numbered  in  opposite  directions  and  having  a 
common  "0"  are  used.  These  verniers  so  used  together  are 
called  Double  Verniers.  Each  part  of  the  double  vernier  will 
equal  n  --  1  divisions  on  the  main  scale,  and  there  will  be  one 


line  in  each  part  that  will  coincide.     Fig.  95  shows  a  double 
vernier. 

To  read  a  Double  Vernier:  The  "0"  on  the  vernier  will  give 
the  smallest  reading  of  the  main  scale  the  same  as  in  a  single 
direct  vernier :  to  get  the  vernier  reading,  look  along  that  part 
of  the  double  vernier  which  is  numbered  in  the  same  direction  as 
the  main  scale  which  is  being  used  is  numbered,  for  the  line  that 
coincides.  Do  not  use  that  part  of  the  vernier  which  is  num- 


216 


Military  Topography  and  Photography 


bered  in  the  direction  opposite  to  which  the  main  scale  being 
used  is  numbered. 

FOLDED  VERNIERS.  A  Folded  Vernier  is  a  single  direct  ver- 
nier that  may  be  read  in  either  direction.  This  is  accomplished 
by  numbering  the  center  division  line  "0"  and  numbering  in 
both  directions  from  the  "0."  Fig.  96  shows  a  folded  vernier. 


Folded  verniers  are  commonly  used  on  the  vertical  scales  of 
transits,  where  an  economy  of  space  is  desired  and  both  plus 
and  minus  vertical  angles  are  read. 

To  read  a  Folded  Vernier:  The  "0"  of  the  vernier  shows  the 
least  reading  of  the  main  scale  as  in  all  other  cases :  to  get  the 
vernier  reading,  look  along  the  vernier  from  the  "0"  in  the 
same  direction  as  the  main  scale  being  used  is  numbered  for 
the  vernier  line  that  coincides.  If  no  such  line  is  found  between 
the  "0"  and  that  end  of  the  scale,  glance  to  other  end  of  the 
vernier  and  look  along  the  same  in  the  same  direction  as  before 
to  the  line  that  coincides,  the  division  lines  being  considered 
numbered  consecutively  upward  until  the  "0"  is  again  reached. 
In  a  folded  vernier,  as  in  case  of  .a  single  direct  vernier,  only 
one  line  coincides. 

ADJUSTMENTS  OF  THE  LEVEL 

1ST  ADJUSTMENT.  TO  MAKE  THE  LINE  OF  SIGHT  DE- 
TERMINED BY  THE  INTERSECTION  OF  THE  CROSS  WIRES 
COINCIDE  WITH  THE  OPTICAL  AXIS  OF  THE  TELESCOPE. 

Test:  Fix  the  intersection  of  the  cross  wires  on  some  definite*  point 
such  as  a  nail  head  on  the  side  of  a  house  about  50  feet  away.  Revolve 
the  telescope  in  its  wyes  until  the  attached  level  is  on  top.  Should  the 
intersection  of  the  cross  wires  remain  on  the  nail  head,  the  line  of  sight 
and  optical  axis  coincide:  if  it  does  not,  the  two  do  not  coincide. 

Adjustment:  Bring  the  intersection  of  the  cross  wire  half  way  to  point 
selected  by  means  of  the  bottom  and  top  cross-wire  reticle  screws,  and 
the  other  half  by  means  of  the  leveling  screws.  Repeat  test  and  adju&- 
ment  until  intersection  of  the  cross  wire  remains  on  point.  The  vertical  cross 
wire  can  be  similarly  adjusted  by  first  turning  telescope  on  one  side  in  its 
wyes  and  then  on  the  other. 


Military  Topography  and  Photography  217 

2ND  ADJUSTMENT.  TO  MAKE  THE  AXIS  OF  THE  ATTACHED 
LEVEL  PARALLEL  WITH  THE  OPTICAL  AXIS  OF  THE  TELE- 
SCOPE. 


^"C  .^tm 


*SECTIONAL  VIEW  OF  WYE  LEVEL 

Clamp  vertical  axis  of  level  and  bring  level  bubble  to  center  of 
level  tube.  Now  lift  the  telescope  out  of  its  wyes,  and  place  it  back  with 
ends  reversed.  Should  the  bubble  not  be  in  the  center  of  level  tube,  the 
axis  of  the  level  is  not  parallel  with  the  optical  axis  of  the  telescope. 

Adjustment:  Bring  the  bubble  half  way  back  to  center  by  means  of 
the  capstan  screws  that  hold  it  to  the  telescope  barrel  and  the  other  half  by 
means  of  the  main  leveling  screws.  Repeat  test  and  adjustment  until 
bubble  remains  in  the  center  upon  reversing  telescope  end  for  end  in  its 
wyes. 

3RD  ADJUSTMENT.  TO  MAKE  THE  AXIS  OF  THE  WYES 
PERPENDICULAR  TO  THE  VERTICAL  AXIS  OF  THE  TELE- 
SCOPE. 

Test:  Bring  the  telescope  over  two  opposite  leveling  screws  and  bring 
bubble  to  center  by  means  of  the  main  leveling  screws:  do  the  same  with 
the  telescope  over  the  other  pair  of  leveling  screws.  With  the  bubble  in 
the  exact  center  in  the  latter  operation,  revolve  the  level  on  its  vertical  axis 
180°.  Should  the  bubble  leave  the  center,  the  wyes  axis  and  the  vertical 
axis  are  not  perpendicular  to  each  other. 

Adjustment:  Bring  the  bubble  half  way  back  to  center  by  means  of 
the  wyes  capstan  screws  which  attach  them  to  the  horizontal  arm,  and  the 
other  half  by  means  of  the  main  leveling  screws. 

ADJUSTMENTS  OF  THE  TRANSIT 

The  adjustments  of  the  plane  table  are  practically  the  same  as  those 
for  the  transit.  Where  the  transit  is  revolved  in  azimuth  180°,  a  line  is  first 
drawn  entirely  around  the  alidade  ruler,  and  the  alidade  telescope  revolved 
180°  in  azimuth,  by  picking  it  up  and  reversing  end  for  end,  putting  it 
back  in  the  rectangle  previously  drawn  around  the  alidade  ruler.  In  all 
adjustments  of  the  plane  table  the  alidade  should  be  in  the  center  of  the 

*Courtesy  of  W.  &  L.  E.   Gurley. 


218 


Military  Topography  and  Photography 


board  and  the  length  of  the  alidade  ruler  over  a  pair  of  opposite  leveling 
screws. 

1ST  ADJUSTMENT.  TO  MAKE  THE  HORIZONTAL  PLANE  OF 
THE  PLATE  LEVELS  PERPENDICULAR  TO  THE  VERTICAL 
AXIS  OF  THE  TRANSIT. 

Test:  Bring  a  plate  level  directly  parallel  with  the  vertical  plane  of  two 
opposite  leveling  screws,  and  bring  the  level  buhble  to  the  center ;  bring  the 
bubble  of  the  other  plate  level  to  the  center  by  means  of  the  other  pair  of 
leveling  screws;  revolve  transit  180°.  Should  the  bubble  leave  the  center  of 
the  latter  plate  level  it  is  out  of  adjustment. 


^SECTIONAL  VIEW  OF  TRANSIT  SOCKETS  AND  CIRCULAR  PLATES 


Adjustment:  Bring  the  bubble  half  way  back  to  center  of  level  tube  by 
means  of  the  capstan  nuts  at  its  ends,  and  the  other  half  by  means  of  the 
main  leveling  screws.  Make  same  test  and  adjustment  for  the  other  plate 
level;  repeat  test  and  adjustment  on  each  plate  level  alternately  until  the 
bubbles  remain  in  the  center  with  the  transit  revolved  to  any  position. 

2ND  ADJUSTMENT.  TO  MAKE  THE  LINE  OF  SIGHT  PER- 
PENDICULAR TO  THE  HORIZONTAL  AXIS  OF  THE  TRANSIT 
TELESCOPE. 

Test:  The  transit  having  been  set  up  and  leveled,  drive  a  stake  in  the 
ground  200  or  300  feet  away  and  in  the  stake  drive  a  pin;  set  the  intersec- 
tion of  the  cross  wires  on  this  pin;  clamp  both  upper  and  lower  limbs 
and  plunge  the  telescope  on  its  horizontal  axis;  sighting  through  the 
telescope^  inverted,  have  a  stake  driven  the  same  distance  away  as  before, 
the  center  of  the  stake  being  at  about  the  intersection  of  the  cross  wires; 
have  a  man  to  stick  a  pin  in  this  stake  at  the  exact  intersection  of  the 
cross  wires ;  loosen  upper  limb  and.  revolve  transit  until  the  cross  wires  are 


*Courtesy  of  W.  &  L.  E.   Gurley. 


Military  Topography  and  Photography 


219 


again  exactly  en  the  first  pin;  clamp  upper  limb  and  plunge  the  telescope. 
If  the  cross  wires  do  not  fall  exactly  upon  the  second  pin  again,  the  cross 
wires  are  out  of  adjustment.  See  Fig.  97. 


FIG.  97 

3RD  ADJUSTMENT.  TO  MAKE  THE  HORIZONTAL  AXIS  OF 
THE  TRANSIT  TELESCOPE  .PERPENDICULAR  TO  THE  VERTI- 
CAL AXIS  OF  THE  TRANSIT. 

Test:  The  preceding  adjustment  having  been  made,  center  the  cross 
wires  on  a  point,  as  a  nail  head,  at  the  top  of  a  house;  depress  telescope 
on  its  horizontal  axis  until  the  bottom  h'ne  of  the  house  is  seen,  such  as  the 
water  table;  have  a  man  to  mark  the  point  where  the  intersection  of  the 
cross  wires  falls;  plunge  telescope  and  revolve  transit  about  180°  and  sight 
the  same  point  at  the  top  of  the  house  with  the  telescope  inverted;  depress 


FIG.  98 


the  telescope  until  the  same  bottom  line  of  the  house  is  sighted.  If  the 
intersection  of  the  cross  wires  does  not  fall  on  the  same  point  at  the  bottom 
of  the  house,  the  horizontal  axis  of  the  telescope  is  not  perpendicular  to 
the  vertical  axis  of  the  transit.  See  Fig.  98. 

Adjustment:  Lower  or  raise  the  movable  support  of  the  telescope  axis 
at  one  end  by  means  of  the  proper  screws  until  one  half  the  difference  is 
corrected.  Repeat  test  and  adjustment  until  instrument  is  in  this  adjust- 
ment. 

4TH  ADJUSTMENT.  TO  MAKE  THE  AXIS  OF  THE  LEVEL 
ATTACHED  TO  TELESCOPE  PARALLEL  WITH  LINE  OF  SIGHT. 

Test:  The  preceding  adjustment  having  been  made,  select  a  nearly  level 
piece  of  ground,  and  drive  a  stake  on  each  side  of  the  transit  and  the 


220 


Military  Topography  and  Photography 


same  distance  from  it — about  200  or  300  feet;  depress  telescope  by  means 
of  vertical  tangent  screw  until  bubble  is  in  center  of  level  tube;  read  the 
elevation  on  the  level  rod  held  on  the  stake  towards  which  the  telescope  is 
sighted  and  record;  revolve  telescope  and  sight  other  stake;  bring  bubble 
of  telescope  level  to  center  again  by  means  of  the  vertical  tangent  screw 
and  read  the  elevation  on  the  level  rod  held  on  the  second  stake,  and  record. 
Note  difference  in  elevation  between  the  two  stakes.  Remove  transit 
to  a  slight  distance  beyond  one  stake  in  prolongation  with  both  stakes; 
sight  the  stakes  and  bring  bubble  of  telescope  level  to  the  center  by  means 
of  the  vertical  tangent  screw;  read  the  elevation  of  both  stakes.  If  the 
difference  in  elevation  is  not  the  same  as  when  the  transit  was  between  the 
stakes,  the  axis  of  the  level  tube  is  not  parallel  with  the  line  of  sight.  In 
Fig.  99,  ac-bd  =  ae-bf,  when  the  axis  of  the  telescope  level  is  parallel  with 
the  line  of  sight. 


FIG.  99    ' 

Adjustment:  Set  the  target  on  the  level  rod  at  "a"  to  read  bf'  -f-  (ac'  — 
bd') ;  sight  the  intersection  of  the  cross  wires  on  the  level  target  at  "a,"  and 
clamp  the  vertical  scale;  with  telescope  clamped,  bring  bubble  of  telescope 
level  to  the  center  by  raising  or  lowering  end  of  the  level  tube  by  means 
of  the  capstan  nuts  at  that  end.  Repeat  test  and  adjustment  from  2nd 
position  until  ac  —  bd  =  ae  —  bf . 

5TH  ADJUSTMENT.  TO  MAKE  THE  "0"  OF  THE  VERNIER 
OF  THE  VERTICAL  SCALE  TO  COINCIDE  WITH  "0"  OF  THE 
VERTICAL  SCALE  WHEN  THE  BUBBLE  OF  THE  TELESCOPE 
LEVEL  IS  IN  THE  CENTER. 

Test:  The  preceding  adjustments  having  been  made,  the  bubble  of  the 
telescope  level  is  brought  to  the  center  by  means  of  the  vertical  tangent 
screw.  Should  the  "0"  of  the  vernier  not  coincide  with  the  "0"  of  the 
vertical  scale,  the  vernier  is  out  of  adjustment. 

Adjustment:  With  the  bubble  of  the  telescope  level  in  the  center  and  the 
telescope  clamped,  loosen  the  screw  at  one  end  of  the  vernier  and  tighten 
the  screw  at  the  other  end  in  the  direction  in  which  it  is  desired  to  move 
the  vernier,  until  the  "O's"  coincide. 

The  adjustments  of  both  the  level  and  transit  are  given  in  the  order  in 
which  they  must  be  made.  This  order  is  very  logical  and  if  the  reasons  for 
the  same,  which  are  very  apparent,  be  thoroughly  understood,  the  sequence 
will  seem  natural  and  the  adjustments  made  as  a  matter  of  habit,  rather 
than  trying  to  remember  them  arbitrarily  from  the  numerical  designations. 


Military  Topography  and  Photography  221 

In  addition  to  the  above  adjustments,  there  are  a  few  others,  which 
must  be  made. 

PARALLAX.    This  must  be  eliminated  every  time  an  angle  is  read. 

Test:  With  the  cross  wires  and  object  in  focus,  move  the  position  of 
the  eye  to  the  right  and  left ;  should  there  also  be  a  movement  of  the  image, 
the  image  is  not  in  the  same  plane  as  the  cross  wires,  and  there  is  parallax, 
as  it  is  called. 

Adjustment:  Point  the  telescope  towards  the  sky  and  bring  the  cross 
wires  in  as  perfect  focus  (definition)  as  possible  by  moving  the  eye  piece 
in  or  out.  Now  sight  the  object,  focus  by  moving  object  glass  in  or  out, 
and  observe  if  there  is  any  parallax.  If  so,  slightly  change  position  of 
object  glass  by  moving  in  or  out.  If  this  does  not  eliminate  the  parallax, 
repeat  entire  test  and  adjustment  until  it  is  eliminated.  The  telescope 
must  be  adjusted  for  parallax  for  each  person. 

To  MAKE  THE  VERTICAL  CROSS  WIRE  PERPENDICULAR  TO  THE  HORIZONTAL 
Axis  OF  THE  TELESCOPE.  This  adjustment  also  makes  the  horizontal  cross 
wire  parallel  with  the  horizontal  axis  of  the  telescope.  This  adjustment 
should  be  made  in  conjunction  with  the  2nd  adjustment  above. 

Test:  Bring  the  top  of  the  vertical  wire  on  a  point  about  50  feet  away, 
and  clamp  the  upper  and  lower  limbs  of  the  transit;  raise  telescope  by 
means  of  the  vertical  tangent  screw,  at  the  same  time  looking  through  the 
telescope  to  see  if  the  vertical  wire  remains  on  the  point  while  the  telescope 
is  being  raised.  If  not,  the  vertical  wire  is  not  perpendicular  to  the  horizon- 
tal axis  of  the  telescope. 

Adjustment:  Loosen  two  adjacent  cross-wire  reticle  screws,  and  tap 
one  of  the  reticle  screws  in  the  direction  in  which  it  is  desired  to  turn  the 
reticle  (for  erecting  telescope).  By  looking  through  the  telescope  the 
amount  can  be  closely  judged.  Repeat  test  and  adjustment,  until  the 
vertical  wire  remains  on  the  point  throughout  its  length  during  the  raising 
or  lowering  of  the  telescope. 

Should  there  be  play  in  the  bearings,  the  transit  cannot,  of  course,  do 
accurate  work ;  should  such  faults  appear  it  would  be  better  for  the  average 
instrument  man  to  send  his  instrument  into  the  shops.  Also  if  it  is  desired 
to  straighten  a  magnetic  needle  or  center  the  pivot  support  of  a  magnetic 
needle. 

The  brass  capstan  adjusting  screws  and  huts  should  not  be  tightened  too 
much,  the  general  rule  being  that  the  pressure  applied  to  them  by  means  of 
the  wire  pin  should  be  felt,  and  there  should  be  no  play.  These  things  should 
be  learned  by  the  beginner  from  an  experienced  man. 

A  topographer  must  know  how  to  take  care  of  and  adjust  all  surveying 
instruments,  the  subject  will  not,  however,  be  further  treated  in  this  book 
for  it  is  thoroughly  covered  in  all  manuals  on  plane  surveying. 

SPIRIT  LEVELING 

As  it  is  presumed  that  the  reader  has  a  knowledge  of  survey- 
ing, the  procedure  in  leveling  will  similarly  be  taken  up  only 
in  a  general  way.  If  a  more  thorough  discussion  of  its  theory 
be  desired,  a  standard  textbook  on  plane  surveying,  or  the 
Engineer's  Field  Manual,  should  be  consulted. 


Military  Topography  and  Photography 


223 


GENERAL  DISCUSSION.  A  line  of  levels  is  run  to  determine 
either  the  difference  in  elevation  between  two  distant  points, 
or  the  elevations  of  a  series  of  points  along  a  line.  The  former 
is  called  differential  leveling;  the  latter,  pro  fie  leveling.  In 
addition  to  these  two  kinds  of  leveling,  there  is  a  third,  called 
reciprocal  leveling,  in  which  between  the  same  two  points  the 
level  is  set  up  (1)  near  one  of  these  points,  and  then  set  up  (2) 
about  the  same  distance  from  the  other  point,  sights  being 
taken  on  the  two  points  at  both  set-ups ;  the  mean  of  the  two 
set-ups  is  taken  as  the  difference  in  elevation  between  the  two 
points. 

In  precise  leveling  the  distance  to  any  back  sight  and  to  any 
frjont  sight  at  any  set-up  should  be  about  equal.  If  the  terrain 
at  any  place  prevents  such  equal  distances  being  taken — such 
as,  sights  across  rivers,  etc.,  reciprocal  leveling  should  be  em- 
ployed between  the  turning  points  on  the  border  of  such  places. 

LEVELING  TEJIMS.  Datum:  In  all  leveling,  the  elevations  of 
all  points  are  referred  to  some  base  level  line  or  plane,  com- 


*WYE  LEVEL 

monly  called,  datum  line  or  datum  plane,  whose  elevation  is 
assumed  to  be  "0."  The  datum  plane  is  usually  sea  level  when 
it  is  known.  When  the  elevation  above  sea  level  of  a  point  on 

*Courtesy  of  W.  &  L.  E.  Gurley. 


224  Military  Topography  and  Photography 

or  near  a  line  of  levels  is  unknown,  the  assumed  datum  or  eleva- 
tion of  the  initial  point  will  be  such  that  no  point  along  the  line 
of  levels  will  be  below  the  assumed  datum  plane.  For  a  line  of 
levels  run  to  any  distance  the  datum  plane  will  really  be  a 
spheroid  of  revolution  determined  by  the  sea  level,  and  similarly 
a  datum  line,  a  great  circle.  For  very  short  distances,  the 
datum  may  be  considered  an  absolute  level  plane  or  straight 
line. 

Plane  of  Sight:  When  the  level  is  in  adjustment,  and  is  set 
up  and  leveled,  the  optical  axis  of  the  level  telescope  will  deter- 
mine an  imaginary  horizontal  plane  when  the  level  is  revolved 
around  its  vertical  axis.  This  horizontal  plane  is  called  the 
plane  of  sight. 

Height  of  Instrument:  The  height  of  the  plane  of  sight 
above  the  datum  plane  or  line,  is  called  the  height  of  instrument. 
It  should  be  observed  that  the  height  of  instrument  is  not  the 
elevation  of  any  station ;  in  fact,  the  level  is  never  set  up  over 
a  station  whose  elevation  is  to  be  determined.  The  height  of 
instrument  less  the  distance  from  the  ground  to  the  plane  of 
sight,  however,  would  give  the  elevation  of  the  ground  at  the 
point  where  the  level  is  set  up. 

Back  Sight:  A  back  sight  is  a  reading  taken  on  a  level  rod 
held  on  a  station  or  point  of  known  elevation  above  the  datum 
in  order  to  determine  the  elevation  of  the  plane  of  sight,  or 
height  of  instrument.  Thus,  if  the  elevation  of  the  known  sta- 
tion is  900  feet,  and  the  back-sight  reading  is  5  feet,  the  height 
of  instrument  would  be  905  feet. 

Front  Sight:  A  front  sight  is  a  reading  taken  on  a  level  rod 
held  on  a  station  or  point  of  unknown  elevation,  in  order  to 
determine  the  elevation  of  such  station  or  point,  the  elevation 
of  the  plane  of  sight,  or  height  of  instrument,  having  been 
previously  determined  by  a  back  sight  on  a  station  of  known 
elevation:  Thus,  if  the  height  of  instrument  is  905  feet,  and 
the  front-sight  reading  is  10  feet,  the  elevation  of  the  unknown 
station  would  be  895  feet.  A  front  sight  reading  may  be  taken 
on  stations  both  in  front  of  and  behind  the  instrument  station. 

Turning  Point  (T.  P.):  -A  station  upon  which  a  front  sight 
has  been  taken  to  determine  its  elevation  from  one  set-up,  and 


Military  Topography  and  Photography  225 

a  back  sight  from  the  succeeding  set-up  is  taken  to  determine 
the  height  of  instrument  at  that   set-up,  is  called  a  turning 
point.     Bench  Marks   (B.  M.)   are  stations  whose 
elevations  have  been  accurately  determined  and  are 
marked  by  monuments  or  plates. 

Intermediate  Stations:  A  station  upon  which 
only  a  front  sight  is  taken  to  determine  its  elevation, 
and  upon  which  no  back  sight  is  taken  to  determine 
height  of  instrument,  is  called  an  intermediate  sta- 
tion. In  profile  leveling  front  sights  are  taken  on 
all  critical  points  along  the- line  of  levels  to  deter- 
mine their  elevation,  and  all  such  stations  not  used 
as  turning  points  are  called  intermediate  stations. 

Signaling:  The  instrument  man  must  prearrange 
proper  suggestive  signaling  with  the  rodmen  so  that 
he  can  control  the  target  setting  of  the  level  rod. 
The  following  are  in  general  use :  the  hand  above  the 
shoulder,  raise  the  target ;  the  hand  below  the  shoul- 
der, lower  the  target ;  waving  the  hand  slowly  above 
the  shoulder,  raise  the  target  slowly;  waving  it 
slowly  below  the  shoulder,  lower  the  target  slowly; 
holding  the  right  arm  straight  out,  plumb  the  rod 
to  the  right ;  the  left  arm  straight  out,  plumb  to  the 
left;  the  rod  and  target  correct,  bring  both  hands 
together  over  the  head  and  let  them  fall  to  the  side. 

LEVELING.  Organization:  A  level  party  will 
usually  consist  of  at  least  three  men — one  instrument 
man,  one  front  rodman  and  one  rear  rodman.  If 
much  cutting  of  underbrush,  etc.,  is  necessary,  the 
level  party  should  be  increased  by  the  necessary 
axemen. 

Procedure:    The  level  is  set  up  and  leveled,  at  a 
convenient  point  with  reference  to  the  first  station, 
or  bench  mark.     This  set-up  should  be  about  mid- 
way between  the  first  station,  or  bench  mark,  and  "LEVELING  ROD 
the  first  turning  point.     Thereafter  the  instrument 
should   be   set   up   about   midway   between   each   two   turning 
points.     A  back  sight  is  taken  at  each  set-up  to  determine  the 

*Courtesy  of  W.  &  L.  E.  Gurley. 


226 


Military  Topography  and  Photography 


"H.  I.,"  and  a  front  sight  on  the  next  turning  point  "T.  P." 
to  determine  its  elevation,  and  the  procedure  thus  continued  to 
the  end.  If  the  elevations  of  intermediate  stations  arc  to  be 
determined,  front  sights  on  such  stations  are  taken  from  the 
nearest  set-up.  Should  the  terrain  make  it  necessary  to  set  up 
near  a  "T.  P.,"  a  reciprocal  level  between  it  and  the  next  sta- 
tion should  be  made.  All -readings  are  carefully  recorded  in 
the  level  note  book. 


FIELD  NOTES. 


Sta. 
BM.#1 

T.P.#1 
#2 
#3 
#4 

#4 
#5 

#6 
B.M.#2 

Sta. 


Differential  Leveling 
B.S.  -    H.I.  F.S. 

5.  1000.5 

3.6  1003.9  4.7 

2.1  997.4  8.6 

6.2  994.3  9.3 
7.4                  998.5                  3.2 
(Reciprocal  Leveling  between  #4 
6.0  -              997.1                 5.7 

11.2  1003.9  4.5 

9.6  1008.3  5.2 

3.1 


Elev. 


B.S. 


Profile  Leveling 
H.I.  F.S. 


992.6 


Elev. 


Elev. 

1000. 

1000.3 
995.3 
988.1 
991.1 


992.7 

998.7 

1005.2 

Elev. 


la  6.2 

Ib  2.1                  926.9 

T.P.#1  2.8   '               921.5                10.3                                             918.7 

la  4.6                  916.9 

Ib  12.9                  908.6 

Ic  73                 914.3 

T.P.#2  6.3                 913.7               14.1                                            907.4 

BM.#2  2.6                                            911.1 

LIMIT  OF  ERROR.  The  limit  of  error  in  leveling  is  given  by 
the  formula,  E  =  C  V  M,  in  which  E  is  the  error,  M  the  dis- 
tance in  miles,  and  C  a  constant  depending  upon  the  character 
and  requirement  of  the  work.  The  value  of  C,  used  by  the 
U.  S.  Geological  Survey  is  .05  for  ordinary  spirit  leveling, 
and  .02,  for  precise  spirit  leveling. 

STANDARDIZING  OF  TAPE  OR  CHAIN 

A  topographer  in  the  field  will  seldom  have  to  test  a  tape  or 
chain  unless  an  accurate  base  line  is  to  be  measured.  A  tape 


Military  Topography  and  Photography  227 

can  be  standardized  by  sending  it  to  the  Bureau  of  Standards, 
Washington,  D.  C.  Upon  the  return  of  this  standardized  tape, 
a  permanent  base  line  should  be  at  once  established  for  stand- 
ardizing other  tapes,  and  for  testing  them  from  time  to  time. 

Among  other  things  the  data  from  the  Bureau  of  Standards 
will  give  (1)  the  temperature  at  which  the  test  was  made, 
(2)  the  tension  at  which  the  comparison  was  made,  and  (3)  the 
length  of  the  tape  corrected  for  a  temperature  of  62°. 

To  CONSTRUCT  A  BASE  OF  STANDARDIZATION.  A  level  floor 
should  be  selected  if  possible  of  the  length  of  the  tapes  to  be 
tested — 100  feet,  or  100  yards  long.  If  not,  a  level  concrete 
side  walk  may  be  used.  A  floor  within  a  building  is  the  best  as 
the  temperature  can  be  regulated  to  62°  F.  more  easily;  while 
out  of  doors  it  is  necessary  to  take  advantage  of  a  cloudy  or 
overcast  day,  and  make  corrections  for  temperature.  A  level 
surface  eliminates  the  necessity  of  correcting  for  sag. 

A  distance  equal  to  the  length  of  the  tape  is  measured  off  as 
accurately  as  possible.  Over  the  ends  of  this  distance  zinc 
plates  3  or  4  inches  square  are  securely  fastened  on  the  floor, 
with  nails  or  screws,  the  center  of  the  plates  being  placed  over 
the  end  marks.  About  six  inches  from  one  plate,  in  prolonga- 
tion to  the  distance,  an  L-shaped  iron  bar  is  securely  bolted 
onto  the'  floor.  The  upper  arm  of  this  L-shaped  iron  should  be 
about  1  to  13/2  inches  high  and  have  a  quarter  inch  hole  in  it 
about  half  way  up.  Through  this  hole  is  placed  an  iron  rod 
with  a  hook  to  catch  the  end  of  the  tape ;  the  other  end  of  this 
iron  rod  (which  passes  through  the  hole  of  the  L-iron)  is 
threaded  and  has  a  nut  by  means  of  which  the  zero  of  the  tape 
can  be  brought  into  exact  coincidence  with  a  scratch  on  the 
near  zinc  plate. 

At  about  P/o  to  2  feet  from  the  other  end  of  the  measured 
distance  and  in  prolongation  to  it,  is  fastened  another  L-iron 
bar.  The  hook  of  the  iron  rod  used  here  catches  the  handle  of 
a  spring  balance  and  this  in  turn  the  handle  of  the  tape.  The 
nut  is  tightened  until  the  spring  balance  reads  the  tension  at 
which  the  tape  was  standardized  at  the  Bureau  of  Standards. 
A  reading  glass  should  be  used  in  bringing  the  zero  of  the  tape 
into  coincidence  with  the  mark  on  the  first  zinc  plate.  A  scratch 
is  then  made  on  the  second  zinc  plate  in  coincidence  with  the 


228  Military  Topography  and  Photography 

100  foot  mark  on  the  tape.  If  it  is  desired  to  make  this  scratch 
just  100  feet,  or  yards  from  the  scratch  on  the  first  zinc  plate, 
a  steel  ruler  marked  in  50ths  or  lOOths  of  an  inch  should  be 
used  to  make  the  correction.  Two  thermometers  are  attached 
to  the  tape,  one-fourth  the  distance  from  both  ends,  and  these 
thermometers  should  read  62°  F.,  or  correction  must  be  made 
for  contraction  or  expansion  due  to  difference  in  temperature 
from  62°  F.  The  25  and  50  foot  or  yard  divisions  may  at  the 
same  time  be  marked  on  zinc  plates  fastened  to  the  floor  at 
such  places. 

By  a  similar  procedure  unstandardized  tapes  may  be  stand- 
ardized by  using  this  permanently  standardized. base.  It  can 
be  similarly  used  in  testing  standardized  tapes  from  time  to 

DETERMINATION   OF   CONSTANTS   OF  A  TAPE 

The  constants  of  a  tape  or  chain  used  for  making  correc- 
tions in  measuring  distances  are— (1)  th.e  error  due  to  differ- 
ence in  temperatures,  (2)  the  error  due  to  difference  in  pull, 
and  (3)  the  error  due  to  sag. 

To  determine  the  constant  of  expansion  due  to  temperature: 
The  tension  is  kept  constant  while  the  length  of  the  tape  is 
measured  on  a  standardized  base  at  different  temperatures  and 
recorded,  from  which  the  expansion  per  foot  for  an  increase  in 
temperature  for  one  degree  can  be  calculated.  A  tape  100  feet 
long  will  change  about  %  inch  in  length  for  a  change  of  sixty 
degrees  F.  in  temperature. 

To  determine  the  constant  of  expansion  due  to  tension:  The 
temperature  of  the  tape  is  kept  constant  while  the  tape  is  sub- 
ject to  different  tensions  (amount  of  pull),  and  the  length  of 
the  tape  is  measured  and  recorded  at  each  change  in  tension, 
the  measurement  being  made  on  a  standardized  base,  from  which 
the  expansion  per  foot  per  pound  tension  may  be  computed. 
A  tape  100  feet  long  will  stretch  slightly  less  than  1/100  of  an 
inch  per  pound  pull. 

Constant  due  to  sag:  The  equation  commonly  used  for  error 
d        wd2 

due  to  sag  is,  C  = (—    — ),  in  which  C  is  the  excess  in  inches 

24         P 

in  length  of  sagged  tape  over  the  true  distance  between  sup- 
porting stakes ;   d  is  the  distance  in  inches  between  supporting 


Military  Topography  and  Photography 


229 


stakes ;   w  is  the  weight  in  pounds  of  one  inch  of  the  tape ;   and 
P  is  the  pull  in  pounds. 

DETERMINATION  OF  STADIA  CONSTANT 

In  view  of  the  facts  that  stadia  rods  are  often  broken,  and 
transits  are  sometimes  exchanged  while  in  the  field,  it  is  best 
to  make  all  stadia  rods  alike  and  to  have  the  instrument  man 
determine  the  stadia  constant  for  the  transit  which  he  uses, 
rather  than  constructing  a  stadia  rod  especially  for  a  transit. 
A  distance  of  about  1000  feet  is  measured  off  and  divided 
into  sections  of  about  100  feet  each.  The  length  of  these  sec- 
tions need  not  be  exactly  lOt)  feet,  and  in  fact  should  not  be, 
but  their  exact  length  should  be  known  and  recorded. 

The  transit  is  set  up  over  an  end  stake  and  the  transit  shifted 
so  that  the  plumb  bob  is  exactly  over  the  center  of  stake.  The 
stadia  rod  is  held  plumb  over  each  stake,  the  stadia  read  and 
the  reading  recorded.  From  these  readings  and  the  true  dis- 
tance the  stadia  constant  for  the  particular  transit  and  stadia 
rod  is  obtained.  The  following  table  will  show  the  method  of 

determining  the  constant : 

Stadia 

Inst.  Sta.          True  Dist.  Reading 

-(c  +  f) 
101 
199 
300.5 
404 


Difference        Per  Cent 


8 

4 
5 
6 
7 
8 
9 

10 
11 


100 
198 
299 
402 
500 
601 
699 
803 
890 
1010 


1 
1 

1.5 


807 


1015 


.5 

.5 

.5 

4 

.5 

.45 

.5 


Stadia  Constant  =  10  )5.29 


From  this  constant  the  following  table  can  be  constructed 
to  correct  stadia  readings: 


0 

10 

20 

30 

40 

50 

60 

70 

0 

0.0 

10.0 

20.0 

30.0 

40.0 

50.0 

60.0 

70.0 

80.0 

90.0 

100 

99.5 

109.5 

119.4 

129.4 

139.3 

149.3 

159.2 

169.2 

179.1 

189.1 

200 

199.0 

209.0 

218.9 

228.9 

238.  S 

248.8 

258.7 

268.7 

278.6 

288.6 

300 

298.5 

308.5 

318.4 

328.4 

338.3 

348.8 

358.2 

368.2 

378.1 

888.1 

400 

398.0 

408.0 

417.9 

427.9 

437.8 

447.8 

457.7 

467.7 

477.6 

487.6 

500 

497.5 

507.5 

517.4 

527.4 

537.3 

547.3 

556.2 

566.2 

576.1 

586.1 

600 

597.0 

607.0 

616.9 

626.9 

636.8 

646:8 

656.7 

666.7 

676.6 

686.f 

700 

696.5 

706.5 

716.4 

726.4 

736.3 

746.3 

756.2 

766.2 

776.1 

786.1 

800 

796.0 

806.0 

815.9 

825.9 

835.8 

845.8 

855.7 

865.7 

875.6 

885.6 

900 

895.5 

905.5 

915.4 

925.4 

935.3 

945.3 

955.2 

965.2 

975.1 

985.1 

230  Military  Topography  and  Photography 

CONTROL  TRAVERSES 

A  control  traverse  is  as  its  name  indicates  a  traverse  in 
which  the  work  done  is  so  accurate  that  it  may  be  used  as  the 
control  of  other  work  that  may  be  based  upon  it. 

The  transit  having  been  tested  for  adjustments,  and  adjusted 
if  found  necessary,  is  set  up  over  the  first  instrument  station, 
which  is  usually  numbered  "0."  The  set-ups  must  be  very 
accurately  done  at  each  station — the  plumb  bob  brought  ex- 
actly over  the  center  of  the  stake  marking  each  instrument 
station  and  the  bubbles  of  the  level  tubes  brought  exactly  to  the 
center  of  their  tubes.  The  center  of  the  stake  can  be  marked 
by  a  tack  head  or  with  a  cross  made  with  a  lead  pencil. 

The  A-vernier  of  the  transit  is  made  to  read  0°  00',  and  the 
reading  of  the  B-vernier  should  be  180°  00',  but  due  to  eccen- 
tricity of  the  scale,  or  a  slight  play  in  the  vertical  axis  this 
reading  may  be  off  one  or  two  minutes,  which  must  be  allowed 
for,  as  will  be  explained  later  on.  With  the  upper  limb  clamped 
and  the  lower  limb  undamped  the  transit  is  oriented,  either  by 
using  the  magnetic  needle  or  by  taking  a  sun  azimuth.  The 
taking  of  a  sun  azimuth  will  also  be  considered  later  on. 

Now  clamp  the  lower  limb,  unclamp  the  upper  limb,  and  sight 
on  the  center  of  the  stake  of  station  No.  1.  To  do  this,  station 
No.  1  is  occupied  by  the  rodman  with  a  stadia  rod.  The  stadia 
rod  is  first  held  with  its  edge  towards  the  transit,  the  left  front 
corner  edge  being  held  exactly  over  the  center  of  the  stake ;  the 
rodman  standing  directly  behind  the  rod  so  as  to  be  in  a  position 
to  hold  it  as  plumb  as  possible.  When  the  rod  is  in  the  center 
of  the  telescopic  field  of  the  transit,  the  upper  limb  is  clamped 
and  by  means  of  the  upper  tangent  screw  the  vertical  cross 
wire  is  brought  into  coincidence  with  the  left  edge  of  the  rod — 
the  right  edge  as  the  transit  man  faces.  If  the  stadia  rod 
neither  leans  to  the  right  nor  to  the  left,  the  vertical  cross  wire 
will  be  in  contact  with  the  rod  throughout  its  length.  If  it  is 
not,  motion  the  rodman  to  plumb  to  the  right  or  left,  and  then 
turn  the  upper  tangent  screw  to  bring  the  vertical  wire  into 
coincidence  with  the  stadia  rod  again,  repeating  until  the  ver- 
tical wire  coincides  throughout  its  length.  The  top  of  the  stake 
should  be  visible  in  the  bottom  of  the  telescopic  field  when  the 
vertical  wire  is  brought  into  contact  with  the  stadia  rod. 


FIG.  101 


232  Military  Topography  and  Photography 

The  verniers  are  then  read  and  recorded ;  the  rodman  sig- 
naled to  turn  the  face  of  his  stadia  rod  to  the  front,  and  the 
stadia  read.  The  center  horizontal  line  brought  to  the  same 
elevation  on  the  stadia  rod  as  the  height  of  the  transit  above 
the  No.  0  stake,  and  the  vertical  angle  is  read.  To  determine 
the  height  of  the  transit,  a  small  rod  six  feet  long  is  carried  by 
the  transit  man.  Read  the  needle,  which  should  always  be  done 
as  a  check  against  mistakes. 


*  RECONNAISSANCE  TRANSIT 

Unclamp  lower  limb,  loosen  the  tripod  screws,  lift  the  needle, 
and  set  up  transit  over  station  No.  1.  Center  the  vertical  wire 
on  stake  No.  0,  the  rear  rodman  holding  the  right  front  corner 
edge  on  the  center  of  the  stake.  Clamp  lower  limb,  using  lower 
tangent  screw  to  bring  vertical  wire  into  exact  coincidence. 
Do  not  touch  upper  limb  clamp  or  the  upper  limb  tangent  screw. 
Read  the  stadia  and  vertical  angle  as  a  check  on  the  readings 
from  the  previous  station,  and  read  the  B-vernier  to  see  that  the 
upper  limb  and  tangent  screw  have  not  been  touched  through 

*0ourtesy  of  W.  &  L.  E.  Gurley. 


Military  Topography  and  Photography  233 

mistake.  If  the  B-vernier  does  not  read  the  same  as  when  at 
station  No.  0,  make  it  do  so,  and  bring  the  vertical  wire  into 
coincidence  by  using  the  lower  limb  and  tangent  screw.  Record 
readings,  the  B-vernier  being  the  control  vernier  at  this  station. 

Unclamp  upper  limb  and  sight  on  stake  No.  2,  read  and 
record  both  verniers,  the  vertical  angle  and  the  stadia  reading 
and  record  them.  Do  not  touch  lower  limb  or  the  lower  limb 
tangent  screw.  Unclamp  upper  limb  and  take  side  shots,  read- 
ing only  the  control  vernier,  the  vertical  angle  and  the  stadia. 
Sight  back  on  station  No.  2,  to  see  if  the  verniers  read  the  same 
as  when  first  sighted  on  No.J2.  If  they  do  not,  the  lower  limb 
or  its  tangent  screw  has  been  touched. 

It  will  be  noticed  that  the  A-vernier  at  station  No,  0  was 
the  control  vernier,  and  the  B-vernier  at  station  No.  1.  When 
the  transit  is  set  up  at  station  No.  2  the  A-vernier  will  be  in 
control  again,  and  so  on  alternately;  the  A-vernier  being  in 
control  at  even  numbered  and  the  B-vernier  at  the  odd  num- 
bered stations.  In  the  record,  the  vernier  in  control  may  be 
indicated  by  the  proper  letter  in  each  case.  This  alternation  of 
control  verniers  eliminates  errors  due  to  eccentricity  of  the 
scale  limb. 

The  following  signs  can  be  conveniently  used:  Right  hand 
straight  out  to  the  right,  plumb  to  the  right ;  left  hand  straight 
out  to  left,  plumb  to  the  left ;  hands  waved  once  above  the  head, 
turn  face  of  stadia  rod  towards  transit ;  hands  waved  twice  over 
head,  put  rod  down ;  to  come  in,  signal  assembly.  Other  signals 
may  be  improvised.  A  plumb  bob  and  line  attached  to  the 
stadia  rod,  is  very  useful  to  plumb  by. 

When  the  A-  and  B-vernier s  are  not  exactly  180°  00'  apart, 
there  is  a  constant  error  that  must  be  corrected ;  if  there  is  an 
eccentricity  in  the  horizontal  scale  this  error  will  be  variable. 
In  either  case  take  one-half  the  error  and  add  it  to  the  vernier  in 
control  (see  record).  The  stadia  reading  and  vertical  angle  are 
read  in  both  directions  between  two  stations,  and  the  mean  taken 
as  the  most  probable  true  value.  It  may  seem  unnecessary  to 
observe  all  these  details  in  a  control  traverse,  but  a  traverse  of 
only  20  to  30  stations  will  bring  home  to  one  the  need  of  the 
most  accurate  work  at  all  times.  The  needle  should  be  read  at 
each  set-up  to  furnish  a  check  on  the  reading  of  the  verniers. 


234 


Military  Topography  and  Photography 


FIELD  NOTES,  CONTROL  TRAVERSE 
Azimuth 


Fm. 

To 

Hor.  D. 

Control 

Check 

Ver.  Aug. 

Elev. 

0 

1 

823 

Ai7ri^ 

350°12' 

-f  1°15' 

710 

1 

0 

823 

J$350°12' 

1°15' 

728 

1 

2 

780 

196°45' 

16°45f 

4-   40' 

737 

2 

1 

780 

A  16°  45' 

—   40' 

a 

615 

75°16' 

4-  5°18' 

793.8 

b 

937 

261°  '43' 

4-  1*3^ 

739.5 

2 

3 

600 

160°40' 

340°40' 

—   15' 

735.4 

3 

2 

600 

'B340°40' 

-f   15' 

3 

4 

580 

221°  45' 

41°  43' 

—    5' 

734.5 

4 

3 

580 

A  41*44' 

+  '  5' 

4 

5 

464 

235°30' 

75°30' 

—   12' 

732.9 

5 

4 

464 

B  75°30r 

4-   12' 

5 

6 

650 

271°  '45' 

19°  45' 

—   55' 

722.5 

6 

5 

650 

A  91*45' 

+   55' 

6 

7 

650 

331°  15' 

151°  15' 

1°10' 

709.3 

7 

6 

650 

H$151°15' 

_j_  i°i(y 

7 

8 

560 

17°50' 

197°50' 

+    4' 

710 

8 

7 

560 

A197°50' 

—   4' 

8 

9 

630 

348°55' 

168°55' 

+   9' 

711.6 

9 

8 

630 

B168°55' 

—    9' 

9 

10 

620 

1°3Z 

181°32' 

+   14' 

714.1 

10 

9 

620 

A181°32r 

—   14' 

10 

11 

520 

27°55' 

207°55' 

-f-   22' 

717.4 

11 

10 

520 

K207°55' 

—   22' 

11 

12 

636 

103°00f 

•  283°00' 

—   33' 

711.3 

12 

11 

636 

A283°00' 

+   33' 

12 

0 

708 

78°45' 

258°45' 

—   15' 

708.2 

0 

12 

708 

15258°  45' 

4   15' 

0 

1 

171*  If 

351°  11' 

Military  Topography  and  Photography  235 


Sketcher Date .' 

Inst.  Man Transit 

Recorder Sta.  Con 

Bearing.  Remarks. 

N  8°45'W  "O"  —  710  Elev. 

N16°45'E 

Side  Shot. 
Side  Shot. 
N19°15'W 

(Control  Traverses  are  plotted  from  the 

NA1°A5'F  coordinates  of  their  stations.  To  do  this  a 

base  point  is  so  selected  that  the  coordinates 

N55°30'E  °^  no  Cation  will  be  "0."  For  Northern 

Latitudes  and  Western  Longitude,  this  will 

S88°15'E  ke  tne  *ower  right-hand  corner  of  the  plot- 

ting sheet.  All  stations  are  plotted  with 

8  28°A5'E  reference  to  this  base  point.  The  coordi- 

nates of  the  traverse  stations  are  computed 

S  17°A5'W  out  an{*  adJusted  on  a  Traverse  Sheet.  The 

traverse  sheet  on  the  following  page  is  of 

S  11°00'E  these  notes.  Coordinates  of  Base  Point, 

40°00'  N.  Lat.,  and  100°00'  W.  Long.;  of 

8  1°30'W  Sta.  "O,"  40°02'  1518  ft.  N.  Lat.,  and!00°03 

3113  ft.  W.  Long.;  or  "Northing,"  17117  ft.— 

S28°99'W  "Westing,"  13622  ft. 

N77°00'W 
8  78° A5' W 


The  diiference  between  North  and  South  Latitudes,  which  should  not  be 
greater  than  1  to  300,  is  divided  in  two  and  each  half  proportioned  equally 
to  its  respective  stations. 

The  Latitude  of  a  course  is  equal  to  its  length  times  the  cosine  of  its 
bearing;  the  departure,  the  length  times  the  sine  of  its  bearing.) 


236 


Military.  Topography  and  Photography 


TRAVERSE  SHEET 
Azimuth ;        Latitude  Departure 


Coordinates 


Fm 

.   To 

Distance;     -{- 

-f 

Bearing.  North 

South          East        West    North'gs 

West'gs 

0 

1 

171°12'      *(Jr.S) 

(  —  2)        (—.5)      (+.5)      17117.0 

13662.0 

823 

813.6 

1264 

N  8°48'W  813.3 

1264      17930.6 

137884 

1 

2 

196°45' 

780 

7473 

224.3 

N16°45'E    746.9 

224.8                        18677.8 

13564.1 

2 

3 

160°  40' 

600 

556.5 

199.2 

N19°20'W  556.2 

198.7      19234.3 

13763.3 

3 

4 

221°  '45' 

580 

433.1 

385.7 

N41°45'E    432.8 

3863                        196674 

13377.6 

4 

5 

235°30' 

464 

263.1 

381.9 

N55°30'E   262.8 

382.4                        19930.5 

12995.7 

5 

6 

271°45f 

650 

19.7 

649.2 

888°15'E 

19.9          649.7                        19910.8 

12346.5 

6 

7 

331°  15' 

650 

569.7 

312.1 

828°45'E 

569.9          312.6                        19341.1 

120344 

7 

8 

17°50' 

560 

532.9 

172.0 

8  17°  50'  W 

533.1                           171.5      18808.2 

122064 

8 

9 

348°55' 

630 

618.0 

120.6 

8  11°  05'  E 

618.2          121.1                        181903 

12085.8 

9 

10 

1°32' 

620 

619.6 

17.1 

8  1°32'W 

619.8                            16.6      17570.6 

12102.9 

10 

11 

27°55' 

520 

4593 

244-0 

827°  55'  W 

4594                           243.5      171114 

12346.9 

11 

12 

103°00' 

636 

1434 

620.2 

N77°00'W  143.1 

619.7      17254.8 

12967.1 

12 

0 

78°45f 

708 

137.9 

694.9 

87  8°  45'  W 

138.1                           6944      17116.9 

12662.0 

2955.1 

29584         2076.8     2070.3 

2955.1         2079.3 

Difference  by  i/2 

3.3               6.5 

1.7               3.3 

*1.7  —  6  =  .5  =  Plus  correction  for  each  North  Latitude. 

1.7  —  7  =  3  =  Minus  correction  for  each  South  Latitude. 

3.3  —  6  =  .5  =  Minus  correction  /or,  each  East  Departure. 

3.3  —  7  =  .5  =  Plus  correction  for  each  West  Departure. 


Military  Topography  and  Photography  237 

BACK  SIGHT  TRAVERSES 

Back  sight  traverses  are  executed  in  the  same  manner  as  con- 
trol traverses,  except  that  they  are  not  closed  on  themselves  for 
check.  The  orientation  of  the  transit  is  carried  forward  in  the 
same  manner  as  in  control  traverses.  Back  sight  traverses  are 
much  more  accurate  than  needle  traverses,  but  require  more 
time.  Back .  sight  traverses  are  not  corrected  for  probable 
errors,  and  generally  plotted  directly  on  the  sketching  board. 
The  record  is  kept  in  the  same  manner  as  for  control  traverses. 

NEEDLE  TRAVERSES 

In  needle  traverses,  the  transit  is  oriented  at  each  set-up  by 
means  of  the  magnetic  needle.  When  oriented,  the  lower  limb 
is  clamped  and  remains  clamped  throughout  the  rest  of  the 
set-up.  If  there  is  a  magnetic  declination,  the  magnetic  declina- 
tion correction  scale  is  so  adjusted  that  when  the  needle  points 
at  its  "0,"  the  horizontal  scale  of  the  transit  is  in  true  azimuth. 
In  passing  from  one  station  to  another  the  magnetic  needle 
should  always  be  lifted  from  its  support.  Needle  traverses  are 
rarely  closed  and  adjusted,  and  in  topographical  surveying  are 
used  mainly  for  filing-in  work. 

To  orient  with  needle:  The  A- vernier  is  set  at  180°  00'  and 
the  upper  limb  clamped ;  the  needle  is  lowered  onto  its  support ; 
the  lower  limb  is  undamped  and  the  transit  turned  to  a  position 
where  the  needle  points  at  "0,"  using  the  lower  tangent  screw 
for  fine  adjustment.  A  transit  so  oriented,  where  there  are  no 
local  magnetic  disturbances,  should  be  within  five  minutes  of 
the  true  north,  or  within  a  total  error  range  of  not  more  than 
10  minutes. 

f  FIELD  NOTES  OF  A  NEEDLE  TRAVERSE 

Fm.     To     Hor.  Dist. 

0  a  430 

b  a  420 

b  c  446 

d  c  320 

d  e  860 

f  e  420 


Azimuth 

Control 

Check     V.  A. 

Elev. 

Bearing 

0=710 

214°35' 

-f  1°00' 

717.5 

N34°SO'E 

200°00r 

4-    4^f 

723 

N40°00'E 

228°30' 

-J-       30' 

727 

N48°30'E 

210°83f 

—      15' 

725.7 

N80°SO'E 

175°30' 

—      IS' 

724.4 

N  4°SO/W 

155°10' 

—      58' 

719 

N24°45'W 

238  Military  Topography  and  Photography 

MEASUREMENT  OF  BASE  LINE  (1:100,000) 

The  selection  of  a  sight  for  a  base  line  has  already  been  dis- 
cussed. (See  page  54.) 

The  two  ends  of  a  precisely  measured  base  line  should  be 
marked  by  concrete  hubs  placed  sufficiently  deep  to  prevent 
movement  by  probable  agencies.  Having  marked  the  ends,  a 
transit  is  set  up  over  one  of  them ;  the  transit  is  sighted  on  the 
distant  hub  and  both  upper  and  lower  limbs  of  the  transit  are 
clamped.  Stakes  are  now  driven  20  feet  apart,  "shooting" 
them  in  line  by  means  of  the  transit  telescope,  and  measuring 
the  distance  apart  by  stadia.  This  operation  can  be  carried 
on  for  about  1000  feet,  when  it  will  be  necessary  to  move  the 
transit  forward,  to  a  stake  that  has  been  accurately  centered 
in  the  line  of  sight  at  the  preceding  set-up.  The  center  of  all 
stakes  should,  in  fact,  be  in  the  line  of  sight. 

In  using  a  100-foot  tape,  which  will  usually  be  the  tape  used 
in  military  surveys,  every  100  feet  along  the  base  line  should 
be  marked  by  a  large  stake,  about  2"  x  4",  while  the  20-foot 
stakes  between  may  be  about  1"  x  2".  The  top  of  all  these  stakes 
should  be  of  the  same  elevation,  which  can  be  secured  by  using 
the  transit  as  a  level,  while  shooting  in  and  measuring  distances. 
To  secure  this,  the  stakes  are  driven  to  such  a  depth  that  the 
tops  are  all  on  the  same  level;  or  if  it  is  impossible  to  do  this' 
on  account  of  hard  ground,  rocks,  etc.,  the  stakes  may  be  sawed 
off  at  the  proper  place.  It  is  usually  necessary  to  divide  the 
Base  Line  into  sections  of  different  elevations.  In  such  cases 
the  top  of  the  stakes  of  each  section  are  all  on  the  same  level. 
The  division  between  adjacent  sections  of  different  elevations 
is  determined  by  a  plumb  bob  and  line. 

Two  small  lath  nails  are  driven  into  each  of  the  smaller  stakes, 
sufficiently  apart  to  allow  the  tape  to  have  free  play  when 
stretched  between  them.  Zinc  plates  arc  nailed  on  the  tops  of 
the  large  100-foot  stakes.  For  holding  the  "0"  and  100-foot 
ends  of  the  tape,  the  same  kinds  of  holders  as  used  in  stand- 
ardizing of  tape  are  used,  except  they  are  fastened  on  heavy 
planks  several  feet  long,  and  firmly  anchored  into  the  ground 
at  each  set-up.  At  about  .one- fourth  the  distance  from  each 
end  of  the  steel  tape  are  attached  two  small  thermometers  for 
observing  the  temperature  of  the  tape  at  each  reading. 


Military  Topography  and  Photography  239 

"The  "0"  of  the  steel  tape  is  first  brought  over  the  center  of 
the  hub  of  the  near  end  of  the  base  line,  using  a  reading  glass 
to  get  it  there.  The  tape  is  then  pulled  to  a  uniform  tension  of 
12  pounds,  or  of  any  other  standard  tension;  the  distance  from 
the  100- foot  mark  on  the  tape  to  the  center  scratch  on  the  zinc 
plate  of  the  100-foot  stake,  is  measured  by  means  of  a  short 
steel  ruler  divided  into  SOths  of  an  inch,  using  a  reading  glass. 
The  distance  between  the  100-foot  stakes,  the  tension,  and  the 
temperature  are  of  course  recorded  as  announced,  and  are 
sufficient  for  all  corrections. 

Overcast  days  are  the  best  for  base-line  measurements,  but 
such  days  cannot  be  waited  for.  l£  the  above  method  be  carried 
out  carefully  and  precisely,  the  Base  Line  should  be  within  a 
maximum  limit  of  error  of  one  part  in  100,000.  Should  so 
accurate  a  base  line  not  be  desired,  the  degree  of  precision  may 
be  reduced:  stakes  may  be  driven  only  every  50  feet,  or  100 
feet — the  "0"  and  100-foot  ends,  including  the  spring  balance, 
may  be  held  directly  in  the  hands — the  center  of  the  100- foot 
stakes  may  be  marked  with  a  pencil,  reading  glass  dispensed 
with,  etc.,  according  to  the  kind  of  base  line  desired. 

RECORD  OF  BASE  LINE  MEASUREMENTS 

Length  of  Tape  Used:  99.935  feet  at  15  Ibs.  tension  and  62°  F. 
Tension  used=  12  Ibs.  Weight  of  tape  per  inch  —  .003  Ibs.  Distance 
between  stakes  ==  20  ft.  Cross  section  of  tape  =  .01  sq.  in. 

Temperature 

Fm.  To        Correction  Minus  Ther.  No.  1  Ther.  No.  2 

AA  1 

1  2 

2  3 

3  4 
#  * 

99        AB 

Totals: 
Average: 

Mean:  69.62° 

BASE  LINE  COMPUTATIONS.  The  computations  of  the  Base 
Line  whose  measurements  are  given  in  the  preceding  paragraph 
will  now  be  made.  This  Base  Line  contains  one  hundred  100- 
foot  sections,  or  five  hundred  20-foot  sections : 


.253 

70.0° 

70.0° 

.346 

70.2° 

•   70.2° 

.111 

70.3° 

70.5° 

.342 

70.5° 

70.6° 

* 

*           * 

*          *           * 

*          * 

.715 

65.2° 

65.1° 

21.214 

43.186 

6958.3° 

6965.5° 

69.58° 

69.66° 

240 


Military  Topography  and  Photography 


Length  of  Base  Lines: 

A  99.935-foot  tape  used  100  times  =r 
Plus  Corrections  = 


Minus  Corrections  = 


Temperature  Correction  —  CL  (t-f),  in  which  C,  the 
coefficient  of  expansion  is  taken  at  .0000065;  L,  the 
entire  length  of  tape  applied,  9993.5  feet;  and  (t-f  ), 
(69.62°-62°)  7.62°  = 


Tension  Correction  =  PI  -j-  ES,  in  which  P  equals  the 
difference  in  pull  used  and  that  at  which  tape  was 
standardized;  1,  the  length  of  the  tape  in  inches; 
E,  the  modulus  of  elasticity,  28000000;  and  S,  the 
cross-sectional  area  of  tape:  or  (3  X  1200  -f-  28000000 
X  .01.  This  correction  is  in  inches,  and  the  cor- 
rection for  one  tape  length  must  be  multiplied  by 
100.  = 


d    /  wd  \  2 
Sag  Correction   (in  inches)   =   -g^C   p~~)  '  in  wh*cn  ^ 

equals  the  distance  between  supporting  stakes  in 
inches;  w,  the  weight  of  tape  per  inch  in  pounds, 
and  P,  the  pull  in  pounds.  This  correction  must 
be  multiplied  by  500  to  get  entire  sag  in  inches,  and 
divided  by  12  to  reduce  to  feet,  or 


9993.500  Ft. 
21.214    " 


10014.714     " 
43.186     " 

9971.528     " 


.495 


9972.023 


—  .107 


9971.916 


True  length  of  Base  Line  —  9970.416     " 

Ekvation  Correction.  If  the  Base  Line  has  an  altitude 
above  sea  and  it  is  desired  to  reduce  it  to  its  length 
or  projection  at  sea  level,  the  following  equation  is 

L  x  R 

used:     X  =-  --  -  —  —  ,  in  which  X  equals  the  pro- 

R  -f  A 

jected  length  at  sea  level;  L,  the  length  of  the  Base 
Line  as  actually  measured  and  corrected;  R,  the 
radius  of  the  earth  in  feet;  and  A,  the  altitude  of 
the  Base  Line  above  sea  level  in  feet.  Where  a  Base 
Line  consists  of  sections  of  diiferent  altitudes  their 
lengths  'must  all  be  adjusted  for  sea  level,  or  for 
a  common  altitude.  R  may  be  taken  as  6,378,000 
meters. 

Base  Lines  are  generally  measured  at  least  twice,  and  if 
the  accuracy  required  is  1  :1  00000,  the  above  base  line  upon 
remeasurement  should  not  vary  from  9970.416  feet  by  more 
than  .1  feet;  if  1  :10000,  then  by  not  more  than  one  foot. 


Military  Topography  and  Photography  241 

MEASUREMENT  OF  ANGLES  IN  SERIES 

Where  it  is  desired  to  determine  the  azimuths  to  several  points 
from  one  instrument  station,  an/1  it  is  desired  to  have  a  check 
against  mistakes,  etc.,  the  following  method  is  used.  The 
transit  is  set  up  and  oriented  in  true  azimuth.  With  the  lower 
limb  clamped  throughout,  the  azimuths  to  the  several  points  are 
read  in  succession,  the  azimuth  to  the  first  point  being  read 
again  immediately  after  reading  the  azimuth  to  the  last  point. 
If  the  second  reading  of  the  azimuth  to  the  first  point  is  the 
same  as  the  first  reading  to  that  point,  it  can  be  safely  presumed 
that  the  lower  limb  screw  an<3  tangent  screw  have  neither  been 
touched  throughout  the  determinations.  If  they  do  not  read 
the  same,  the  whole  procedure  should  be  repeated  until  they  do. 
To  make  the  readings  more  accurate,  the  telescope  may  be 
plunged  and  the  azimuths  read  in  the  reverse  direction.  This 
latter  method  is  used  when  reading  the  azimuths  of  the  adjacent 
secondary  triangulation  stations  from  a  primary  triangulation 

station. 

RECORD  OF  ANGLES  MEASURED  IN  SERIES 

Azimuth 

From          To        Dist.        Control  Check  V.  A.  Elev. 

Tel.  Direct 
AC  Al 

A2 
A3 
A4 
Al 

Tel  Plunged 
AC  Al 

At 
A3 
A2 
Al 

MEASUREMENT  OF  ANGLES  BY  REPETITION 
In  triangulation  work,  the  angles  of  each  primary  triangle 
are  required  to  be  measured  more  accurately  than  can  be  done 
in  a  single  reading  with  the  common  transit.     In  such  cases 
each  angle  is  measured  by  the  method  of  repetition. 

Assuming  the  transit  to  be  set  up  at  AA  of  a  triangle  whose 
other  vertices  are  AB  and  AC;  AB  being  to  the  left  and  AC 
to  the  ri^ht.  The  A-vernier  is  set  at  0°  00'  with  the  upper 


190° 

45' 

10° 

45' 

—2° 

10' 

275° 

53' 

95° 

53' 

—1° 

41' 

356° 

ir 

176° 

ir 

—2° 

25' 

88° 

30' 

268° 

30' 

—3° 

15' 

190° 

45' 

10° 

45' 

—2° 

10' 

190° 

45' 

10° 

45' 

—2° 

10' 

88° 

30' 

268° 

30' 

—3° 

15' 

856° 

IT 

176° 

17' 

—2° 

25' 

215° 

53f 

95° 

53' 

—1° 

4? 

190° 

45' 

10° 

45' 

—2° 

10' 

242  Military  Topography  and  Photography 

limb  clamped ;  the  transit  is  then  sighted  on  AB,  clamping  the 
lower  limb  as  soon  as  sighted  and  using  the  lower  tangent  screw 
for  fine  adjustment;  with  the  l^ower  limb  clamped,  unclamp  the 
upper  limb  and  sight  AC,  using  the  upper  tangent  screw  for 
fine  adjustment;  read  the  verniers  and  record,  the  reading  of 
the  control  vernier  ( A-vernier)  is  the  si/e  of  the  angle  AAABAC. 

Now  unclamp  the  upper  limb  and  sight  again  on  AB :  if  the 
A-vernier  reads  0°  00'  the  lower  limb  has  not  been  moved  and 
the  size  of  the  angle  is  correct  for  a  single  reading. 

Unclamp  upper  limb,  sight  on  AC  again;  clamp  the  upper 
limb,  unclamp  the  lower  limb,  and  sight  on  AB ;  clamp  the  lower 
limb,  unclamp  the  upper  limb  and  sight  AC.  Repeat  this  opera- 
tion five  times,  and  on  the  last  sight  on  AC,  read  the  vernier 
and  record.  By  this  procedure  the  horizontal  scale  of  the 
transit  has  been  moved  through  five  times  the  angle  ABAAAC, 
and  on  the  last  sight,  the  control  vernier  (A-vernier)  will  read 
five  times  the  angle  BAG.  Should  the  angle  BAG  be  greater  than 
72°,  it  will  be  necessary  to  add  360°  to  the  reading  of  the  con- 
trol vernier;  for  the  scale  reads  only  to  360°. 

Now  plunge  the  telescope  and  read  the  angle  CAB,  and  then 
five  times  angle  CAB,  as  was  explained  in  the  preceding  para- 
graphs. With  the  telescope  plunged  the  B-vernier  will  be  in 
control;  AC  will  be  the  first  sighted  on,  and  AB  second.  The 
size  of  the  angle  will  be  360°  minus  the  reading  of  the  control 
vernier  (B-vernier). 

In  using  a  transit  reading  to  minutes,  the  limit  of  error  will 
be  one  minute  whether  a  single  reading  is  taken  or  an  angle  five 
times  as  great  is  measured.  By  reading  an  angle  by  multiplying 
its  size  by  five,  therefore,  the  limit  of  error  is  reduced  one-fifth, 
or  to  12  seconds  in  this  case.  The  mean  of  the  values  thus 
obtained  is  taken  as  the  most  probable  value  of  the  angle  BAG. 

In  triangulation  work,  all  the  angles  of  a  triangle  must  be 
measured  by  repetition  and  their  sum  should  not  vary  from 
180°  00'  00"  by  15".  If  so,  the  measurement  of  the  angles 
must  be  repeated.  The  error  when  less  than  15  seconds  is  dis- 
tributed equally,  to  the  three  angles  of  the  triangles. 


244  Military  Topography  and  Photography 


RECORD  OF  ANGLES  OF  TRIANGLE  MEASURED  BY  REPETITION 

Fm.         To        Dist.            V.   A.  Control  Check 
Telescope  Direct  —  Angle  ABC: 

AB      AA               +0°  w  o°  oo'  iso0  oo' 

AC                      +3°  36'  48°  10'  %®8°  10' 

5ABC                        .  $40°  W  60°  49' 

Telescope  Inverted  —  Angle  CBA  : 

AB          AC                      +3°  36'  0°  00'  180°  00f 

AA                      +0°  10'  311°  50'  131°  50' 

5CBA  119°  11'  899°  11' 

Telescope  Direct  —  Angle  CAB: 

AA      Ac               +1°  4®'  o°  oo'  180°  oof 

AB                       -0°  10'  75°  19'  255°  19' 

5CAB  16°  3%'  196 


° 


Telescope  Inverted  —  Angle  BAG: 

^A         AB                        -r  10'  0°  00f  180°  00' 

AC                  +1°  4®'  ®n°  41'  44°  41' 

'5BAC  843°  28'-  163°  18' 

Telescope  Direct  —  Angle  EGA  : 

AC         AB                       -3°  36f  0°  00'  180°  00' 

AA                       ~1°  4®'  06°  3%'  %36°  3%' 

5BCA  050°  38'  102°  38' 


Telescope  Inverted  —  Angle  ACE: 

AC      AA                -1°  4%'  o°  oo'  180°  oo' 

AB                      —  3°  36'  303°  $8'  1%3°  %8f 

5ACB  77°  W  257°  W 


Military  Topography  and  Photography  245 


Angle  Mean          Correction     Adjusted  Angle 


48°       9'     48" 


48°       9'     48"          48°      9'    48"    +    4"    =    48°      9 


75°     18 


75°     18'     24"  75°    18'    24"    +    4"    =    75°    18'    28" 


56°     31'     36" 


56°     31'     36"          56°    31'    36"    +    4"    =    56°    31'    40" 
Sum  179°    59'    48"  180°    00'    00" 

Error  12" 


246 


Military  Topography  and  Photography 


AZIMUTH  BY  POLARIS 

PROCEDURE.  The  transit  is  set  up  over  a  stake,  leveled, 
sighted  on  a  second  stake  at  200  or  300  feet  distance,  and  the 
lower  limb  clamped.  The  A-vernier  may  be  set  at  0°  00',  or  at 
the  estimated  azimuth  to  that  stake.  When  Polaris  becomes 
visible  in  the  evening,  the  telescope  of  that  transit  is  sighted  on 
it,  the  intersection  of  the  cross  wires  being  brought  exactly  on 
Polaris.  To  make  the  cross  wires  visible,  a  light  is  held  at  a 
proper  position  in  front  of  the  object  glass  to  illuminate  them. 
If  the  intersection  is  made  at  the  culmination  of  Polaris,  no 
correction  is  necessary  for  the  azimuth  of  Polaris,  but  if  taken 


-ft 


Turn   circle  until  current  month  is  on  top:   the   Great  Dipper   is  then   shown   in   its 
relative  position  at  8:30  p.   M. 


Military  Topography  and  Photography  247 

at  any  other  time  a  correction  is  necessary.  Intersection 
on  Polaris  taken  at  elongation,  corrected  for  the  azimuth  of 
Polaris  at  that  time,  is  perhaps  the  more  accurate;  for  the 
lateral  movement  of  Polaris  is  not  so  rapid  and  the  cross  wires 
can  be  more  easily  centered  on  it. 

Throughout  the  entire  procedure  the  lower  limb  remains 
clamped.  To  sight  Polaris,  the  upper  limb  is  undamped  and  the 
telescope  directed  on  it,  the  upper  limb  is  then  clamped  using 
the  upper  tangent  screw  for  fine  adjustment.  The  angle  at  the 
transit  between  the  distant  stake  and  Polaris,  corrected  for  the 
azimuth  of  Polaris  at  the  time  oi.observation,  added  to  or  sub- 
tracted from  180°00'  according  as  to  whether  the  distant  stake 
is  to  the  east  or  west  of  Polaris,  gives  the  true  azimuth  between 
the  two  stakes. 

AZIMUTH  OF  POLARIS 

Clock  reading  of:  Clock  reading  of: 

Cass.                Ursa          Azimuth  Cass.  Ursa  Azimuth 

Major  of  Major  of 

z  Polaris                  8  z  Polaris 

12.30                   6.30  18'                    6.30  12.30  359°42' 

1.00                   7.00  35'                    7.00  1.00  359°25' 

1.30                   7.30  49'                    7.30  1.30  359°11' 

2.00                   8.00  61'                    8.00  2.00  358°59' 

3.00                   9.00  70'                    9.00  3.00  358°50' 

4.00                 10.00  61'  10.00  4.00  358°59' 

4.30                 10.30  49'  10.30  4.30  359°11' 

5.00                 11.00  35'  11.00  5.00  359°25' 

5.30                 11.30  18'  11.30  5.30  359°42' 

In  this  table  the  azimuth  of  Polaris  at  culmination  is  con- 
sidered 0°00'.  In  most  survey  work,  the  azimuth  of  true  north 
is  considered  180°00',  and  where  so  used  the  azimuths  of  Polaris 
as  given  in  this  table  must  be  corrected  by  adding  180°00' 
to  each  of  them. 

The  above  table  is  for  the  epoch  of  1911,  but  may  be  used 
until  1930.  The  table  is  good  for  latitudes  0°-18°,  for  other 
latitudes  the  following  corrections  must  be  made,  adding  the 
same  azimuths  of  the  left  hand  column  and  subtracting  from 
azimuths  of  the  right  hand  column: 

Lat.  19°-30°,  1/10  Lat,  43°-46°,  4/10  Lat.  54°-57°,  7/10 

Lat.  31°-37°,  2/10  Lat.  47°-50°,  5/10  Lat.  58°-59°,  8/10 

Lat.  38°-42°,  3/10  Lat   51°-53°,   6/10  Lat.  59°-60°,  9/10 


248  Military  Topography  and  Photography 

AZIMUTH  BY  SUN  OBSERVATION 

ASTRONOMICAL  TERMS.  For  the  purpose  of  astronomical 
locations  of  heavenly  bodies,  the  universe  is  represented  by  an 
imaginary  sphere  whose  center  is  the  earth  and  whose  radius 
is  equal  to  the  distance  from  the  earth  to  the  sun.  Similar  to 
those  of  the  earth,  Great  Circles  of  the  celestial  sphere  are  those 
circles  of  that  sphere  whose  plane  passes  through  the  center 


(Zenith) 


(Pole) 


South  S"  K */- *&.— 


Sunrise 


FIG.  103 


of  the  celestial  sphere,  or  what  amounts  to  the  same  thing, 
those  circles  of  the  celestial  sphere  whose  planes  pass  through 
the  center  of  the  earth.  A  Vertical  Circle  is  a  great  celestial 
circle  which  passes  through  both  the  zenith  and  nadir.  The 
Equi-Noctial  circle  (ED)  is  the  celestial  equator.  Hour  Circles 
are  great  celestial  circles  which  pass  through  the  celestial 
poles ;  the  hour  circles  correspond  to  the  terrestrial  meridians  of 
longitude ;  they  are,  however,  numbered  counterclockwise,  from 
"0"  to  "23,"  inclusive.  The  Vernal  Equinox  is  the  hour  circle 
that  is  numbered  "0";  it  is  the  celestial  Greenwich — the 
meridian  on  which  the  sun  crosses  the  equator  on  March  20; 

*In  the  text  North  is  referred  to  as  N;  and  South,  as  S". 


Military  Topography  and  Photography  249 

the  vernal  equinox  passes  through  B  Cassiopeia  (R.  A.,  4  m). 
The  Meridian  (NZS")  is  the  great  circle  which  passes  through 
the  celestial  poles  and  the  zenith.  The  Prime  Vertical  (HZO) 
is  the  great  celestial  circle  which  passes  through  the  zenith 
and  nadir  perpendicular  to  the  Meridian. 

The  Altitude  of  a  star  is  its  angular  elevation  above  the 
horizon ;  the  Zenith  Distance  of  a  star  is  the  angular  distance 
between  it  and  the  zenith;  altitude  +  zenith  distance  =  90°. 
The  Declination  of  a  star  is  its  angular  distance  north  or  south 
of  the  celestial  equator,  measured  along  its  hour  circle;  the 
Polar  Distance  of  a  star  is  its  angular  distance  from  a  celestial 
pole,  measured  along  its  hour  circle ;  declination  -f-  polar  dis- 
tance=  90°.  The  Hour-Angle  of  a  star  is  the  angle  between 
the  meridian  and  the  hour  circle  of  that  star,  measured  from 
the  meridian  eastward  to  the  hour  circle.  The  Right  Ascension 
of  a  star  is  the  angle  between  the  vernal  equinox  and  the  hour 
circle  of  that  star,  measured  clockwise.  The  Azimuth  of  a 
star  is  the  angle  at  the  zenith  between  the  meridian  and  the 
vertical  circle  of  that  star  measured  clockwise  from»the  true 
south;  it  is  also  the  angle  between  the  plane  of  the  meridian 
and  the  plane  of  the  vertical  circle,  which  is,  of  course,  the  same. 
The  Amplitude  of  a  star  is  the  angle  at  the  zenith  between 
the  prime  vertical  and  the  vertical  circle  of  that  star;  azimuth 
±  amplitude  ==  90°  or  270°. 

TIME.  The  Solar  Day  at  any  place  is  determined  by  the  two 
successive  transits  of  the  meridian  at  that  place  by  the  Sun. 
Since  the  motion  of  the  sun  in  right  ascension  is  not  the  same 
throughout  the  year,  but  varies  constantly,  the  length  of  the 
solar  day  throughout  the  year  is  not  constant.  The  Apparent 
Solar  Time  is  the  time  shown  by  a  sun  dial.  Greenwich 
Apparent  Noon  is  the  exact  moment  at  which  the  Sun  transits 
the  Greenwich  meridian.  The  Mean  Solar  Day  is  the  average 
length  of  the  solar  days  of  the  year.  The  differen.ce  in  time 
between  the  apparent  solar  day  and  the  mean  solar  day  is 
called  the  Equation  of  Time  for  that  day,  and  this  equation  of 
time  for  each  day  is  given  in  the  Solar  Ephemeris  Tables.  In 
taking  sun  observations  for  azimuths,  mean  solar  time  must 
be  changed  to  apparent  solar  time. 


250  Military  Topography  and  Photography 

} 

GENERAL  PRINCIPLES.  If  in  Fig.  103,  we  know  the  azimuth 
of  a  star,  or  the  sun,  which  is  S"OH,  or  its  equal  S"ZH,  and  at 
O  measure  this  angle,  or  azimuth,  from  S,  we  shall  have  a  true 
north  and  south  line  ON,  which  is  the  projection  of  the  celes- 
tial meridian  on  the  earth.  In  practice  this  cannot  be  done 
accurately,  for  the  azimuths  of  the  stars  and  the'  sun  are  con- 
stantly changing  throughout  the  day.  An  auxiliary  point  N'  is 
selected  as  near  the  north  as  can  be  estimated :  the  angle  N'OH 
is  taken  at  a  particular  moment,  at  which  moment  the  altitude 
of  the  star  is  also  taken.  From  this  data  the  spherical  triangle 
SZP  is  solved  for  the  angle  PZS,  or  its  equal  NOH,  the 
azimuth  of  the  sun  at  the  instant  the  observation  was  taken. 
It  is  evident  that  N'OH  -  -  NOH  is  the  angular  difference 
between  the  estimated  and  the  true  north.  By  subtracting  or 
adding  this  angular  difference  N'OH,  according  as  to  whether 
NT'  is  to  the  west  or  east  of  true  north,  the  true  position  of  ON, 
or  true  north,  is  obtained.  Corrections  must  be  made  for 
refraction,  etc.,  which  will  be  taken  up  later. 

SOLUTION  OF  THE  ASTRONOMICAL  TRIANGLE.  The  astronomi- 
cal triangle,  SZP,  can  be  solved  by  any  of  the  several  proper 
trigonometric  formulae.  The  following  formulae  are  recom- 
mended. 

sin  (s  —  PN)  sin  (S  —  HS)~ 
Ta"  %Z  =  cosscos(s-SP) 

(COS  8   COS    (S  —  SP) 

Cos  %Z  =     V     cos  PN  cos  HS 

roq  A  .  ±  cos  PS  —  tan  PN  tan  HS;    or, 

cos  PN  cos  HS 

Cot  i/2 A  =      Vsec  s  sec  (s  —  SP)  sin  (s  —  HS)  sin  (s  —  PN). 

Z  is  the  angle  SZP,  or  angle  HON,  or  the  azimuth  of  the 
star  or  sun  from  the  true  north. 

A  is  the  angle  between  the  south  and  the  sun,  measured 
counterclockwise. 

PN  is  the  latitude  of  the  station  of  observation,  and  for  any 
station  is  constant. 

HS  is  the  altitude  of  the  sun,  and  is  measured  directly  by  the 
vertical  scale  of  the  transit :  it  must,  however,  be  corrected  for 
refraction. 


Military  Topography  and  Photography  251 

SP  is  the  north  polar  distance  of  the  sun  at  the  moment  of 
observation:  it  is  obtained  by  adding  or  subtracting  the 
declination  of  the  sun  to  or  from  90°.  The  declination  of  the 
sun  for  Greenwich  apparent  noon  is  obtained  from  ephemeris 
tables. 

s  =  i  (HS  +  PN  +  SP). 

PARALLAX  AND  REFRACTION.  A  line  from  a  star  to  the  center 
of  the  earth  and  a  line  from  the  same  star 'to  the  station 
of  observation  form  a  small  angle,  called  the  parallax  of  the 
star:  the  size  of  this  angle  depends  upon  the  latitude  of  the 
station  of  observation,  but  since  it*is  never  more  than  9  seconds, 
it  may  be  disregarded. 

Refraction  is  the  divergence  of  the  rays  of  light  from  the  sun 
as  they  pass  through  air  of  increasing  density  in  approaching 
the  surface  of  the  earth.  Refraction  is  greater  when  the  rays 
of  light  enter  the  air  at  an  angle,  as  in  the  early  morning  or 
late  afternoon,  and  is  less  when  they  enter  perpendicularly,  as 
at  near  noon.  At  noon,  there  is  only  meridional  refraction. 
Observations  for  sun  azimuth  should  not,  if  it  can  be  avoided, 
be  taken  before  9  A.  M.  and  after  3  :00  P.  M. 

In  the  Nautical  Almanac  and  Ephemeris,  published  by  the 
Naval  Observatory,  will  be  found  tables  giving  the  refraction 
for  one,  two,  three,  and  four  hours  before  and  after  noon  for 
each  day  for  the  40th  latitude  and  factors  of  correction  for 
other  latitudes.  This  correction  is  negative  and  is  subtracted 
from  the  altitude  of  the  sun  as  observed  and  read  from  the 
transit. 

DECLINATION.  The  declination  of  the  sun  at  Greenwich 
apparent  noon  is  given  for  each  day  of  the  year  in  the  Nautical 
Almanac :  the  hourly  change  in  declination  for  each  day  is  also 
given.  From  the  difference  in  time  between  Greenwich  and  the 
station  of  observation,  the  declination  of  the  sun  at  the  moment 
of  observation  can  be  computed.  If  the  station  of  observa- 
tion is  on  the  75th,  90th,  105th  or  120th  meridian  of  longitude, 
the  difference  in  time  will  be  5,  6,  7,  or  8  hours,  respectively, 
from  Greenwich.  Should  the  longitude  be  other  than  these 
meridians,  it  will  be  necessary  to  add  or  subtract  the  proper 
correction  for  difference  in  longitude,  which  is  4  minutes  in 
time  for  each  degree  of  longitude. 


252  Military  Topography  and  Photography 

An  example  in  the  determination  of  declination  will  now  be 
given.  What  was  the  decimation  of  the  sun  at  9:00  A.  M., 
June  15,  1913,  at  a  point  on  the  earth  whose  longitude  is  100° 
West? 

From  the  Ephemeris  Table  the  declination  of  the  Sun  on  this 
day  at  Greenwich  apparent  noon  was  23°18'18.9"  North:  the 
hourly  difference  in  declination  6.75".  The  100th  meridian 
will  be  transited  by  the  sun  6  hours  and  40  minutes  after 
Greenwich  shall  have  been  transited  (100°  X  4'  =  400',  or  6 
hours  and  40  minutes.  At  9 :00  A.  M.,  therefore,  the  sun  will  be 
3  hours  and  40  minutes  past  the  Greenwich  prime  meridian.  In 
3  hours  and  40  minutes,  the  change  in  declination  will  be 
3f  X  6.75"  ==  24.75".  Since  at  this  time  of  the  year  the  Sun 
is  traveling  towards  the  north,  this  correction  must  be  added 
to  the  declination  of  the  Sun  for  Greenwich  apparent  noon  on 
this  date.  23°18'18.9"  +  24.75"  =  =  23°18'43.65",  is  there- 
fore the  declination  of  the  Sun  at  9:00  A.  M.  for  the  100th 
meridian  on  this  date.  The  apparent  time  for  this  date  must 
be  corrected  by  +  7.9"  (from  the  same  table),  which  gives 
23°18'51.55",  as  the  corrected  declination. 

METHOD  OF  OBSERVATION.  The  sun  cannot  be  sighted  v 
through  the  telescope  without  injury  to  the  eyes,  unless  the 
object  glass  is  smoked,  which  is  undesirable.  If,  however,  the 
telescope  is  pointed  at  the  sun,  so  that  the  sun  rays  will  pass 
through  the  telescope,  the  image  of  the  sun  can  be  caught  on  a 
white  screen  or  cardboard  held  five  or  six  inches  from  the  eye- 
piece of  the  telescope.  Images  of  the  cross  wires  will  also  show 
on  the  cardboard,  and  appearance  of  the  cardboard  with  the 
images  will  be  as  shown  in  the  diagrams  in  the  "Record"  below. 

The  transit  must  be  in  adjustment,  and  is  set  up  at  the  point 
desired:  an  auxiliary  point  N',  such  as  a  tree  or  fence  post 
with  a  point  marked  on  it  can  be  selected  as  the  estimated  north 
point.  The  A-vernier  is  set  to  read  180°00' :  the  lower  limb  is 
undamped  and  the  transit  is  sighted  on  the  point  on  N',  using 
the  lower  tangent  screw  for  fine  adjustment:  read  the  A-vernier 
to  see  that  it  still  reads  1SO°00'. 

With  another  man  holding  the  cardboard  to  catch  the  images 
from  the  telescope,  bring  the  Sun's  image  into  the  first  quad- 


Military  Topography  and  Photography  253 

rant,  the  upper  limb  and  vertical  axis  being  undamped :  when 
near  the  intersection  of  the  central  cross  wires,  clamp  both  the 
upper  limb  and  the  horizontal  axis,  and  bring  the  edge  of  the 
sun's  image  in  contact  with  both  wires  simultaneously,  using 
the  upper  tangent  screw  and  the  vertical  tangent  screw.  The 
contact  will  be  for  a  moment  only,  as  the  movement  of  the  sun's 
image  across  the  field  is  perceptible.  Just  before  the  contact, 
the  instrument  man  should  say,  "Ready,"  and  at  the  exact 
instant  "Take."  The  recorder  takes  the  time,  observing  the 
number  of  seconds,  and  records  it.  The  instrument  man  reads 
the  A  and  B  verniers  and  the*"vertical  angle,  the  recorder 
recording  the  readings  as  announced.  Another  observation  is 
taken  with  the  image  of  the  sun  in  the  3rd  quadrant.  The 
transit  should  then  be  sighted  back  on  N'  to  see  that  the 
A-vernier  still  reads  180°00',  and  this  is  done  after  every  set  of 
readings.  It  is  very  easy  to  touch  the  lower  clamp  or  tangent 
screw  by  mistake,  and  if  so,  it  is  necessary  only  to  repeat  the 
work  since  the  previous  check :  otherwise  the  whole  work  must 
be  rejected.  The  telescope  is  then  plunged  and  the  image  of  the 
sun  first  brought  into  the  2nd  quadrant  and  then  into  the  4th 
quadrant.  When  the  telescope  is  plunged,  the  B-vernier  will 
be  in  control.  If,  when  the  telescope  is  plunged  and  sighted 
on  the  point  on  N',  the  B-vernier  does  not  read  180°00'  exactly, 
it  is  better  to  note  the  difference  from  180°00'  and  apply  it  as 
a  correction  to  the  subsequent  readings  of  the  verniers  while 
the  telescope  is  plunged,  rather  than  setting  the  B-vernier  to 
read  180° 00'  and  bringing  the  telescope  onto  the  point  by  using 
the  lower  tangent  screw. 

If  it  is  desired  four  sets  of  readings  may  be  taken.  In  the 
first  set,  the  telescope  is  normal  and  the  sun's  image  is  brought 
into  the  1st  and  3rd  quadrants:  in  the  second  set,  the  telescope 
is  inverted  and  the  sun's  image  is  brought  into  the  2nd  and  4th 
quadrants :  in  the  third  set,  the  telescope  is  inverted  and  the 
sun's  image  brought  into  the  1st  and  3rd  quadrants :  in  the 
fourth  set,  the  telescope  is  normal  and  the  sun's  image  is 
brought  into  the  2nd  and  4th  quadrants.  Where  a  sun  azimuth 
is  taken  in  a  control  traverse  all  four  readings  will  generally  be 
taken  in  rapid  succession.  In  triangulation  work,  however,  the 


254 


Military  Topography  and  Photography 


first  two  sets  of  readings  will  generally  be  taken  in  the  morning, 
while  the  second  two  will  be  taken  in  the  afternoon,  at  about 
an  equal  time  from  noon. 


FIELD  NOTES  OF  SUN  AZIMUTH 

The  Field  Notes  of  a  Sun  Azimuth  should  be  kept  in  the  following 
manner:    (Assumed  azimuth  from  Sta.  O  to  Sta.  N',  180°00'). 


Quadrant 


1st: 
3rd: 
Sums: 
Means : 


*nd: 
4th: 

Sums : 
Means , 


1st: 
3rd: 
Sums : 
Means , 


2nd: 
4th: 

Sums  : 
Mean* 


Latitude : 
Longitude : 
Date: 


Time 
hr.  m.   s. 

10.45.15 
10.47.41 
21.32.56 
10.46.28 


10.51.29 
10.53.33 
21.44.  2 
10.52.31 


10.55.15 
10.58.16 
21.53.31 
10.56.45 


11.02.59 
11.04.21 
22.06.20 
11.03.40 


Azimuth  to  Sun 


Control 


Check 


Set  I,  Direct 
321°30'  141030' 

322°45'  142°45' 

644°15' 
322°   7'30" 

Set  II,  Reversed 
323°23'  143°23' 

323°10'  143°10' 

646°33' 
323°16' 30" 

Set  III,  Reversed 
323°32'  143°32' 

325°00'  145°00' 

648°32' 
324°16' 

Set  IV,  Direct 
326°10'  146°10' 

325°46'  145°46' 

651°56' 
325058' 


Altitude 


27°56' 
27°25' 
55°21' 
27°40'30' 


28°30' 
28015' 
56°45' 
28°22'30' 


28058' 
28°  58' 


28"58 


30°  5' 
29045' 
59°50' 
29°55' 


Bearing 


S46°30'E 

S45°15'E 

90°45' 

S45°22'E 


S44°35'E 
S44°10'E 

88°45' 
S44°22'E 


S44°30'E 
S43°00'E 

87°30f 
S43°45'E 


S41°50'E 
S42°15'E 

84°   5' 
S42°   2'E 


The  "means"  from  the  field  note  sheet  are  transferred  to  a   Sun 
Azimuth  Sheet  and  the     computations  made  thereon. 

28°     40'      00"         N.  From  Station  "O"  to  Station  "N'." 
100°     SO'      00"        W.  Assumed  Azimuth:     ON',  180°  00'. 
December  24,  1915  Watch  5  min.  fast  for  90th  Mer.  T. 


Military  Topography  and  Photography 


255 


j»  :r.  o     *d      P- 

:     ss  r  i 

^w-ie-ltrsja 

gs-o 


SUN  AZIMUTH  COMPUTATION 

II  IS  gg-j'^fr*" M  ^ 


B-fS.' 


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256  Military  Topography  and  Photography 

LATITUDE,  LONGITUDE,  AND  AZIMUTH  DETERMINATION  BY  THE 
SOLAR  ATTACHMENT* 

GENERAL  EXPLANATION  or  THE,  SOLAR  ATTACHMENT.  There 
are  two  solar  attachments,  the  Burt  and  the  Telescopic.  The 
former  is  attached  to  the  telescopic  axis,  and  the  latter,  to  the 
telescopic  standards,  as  shown  in  the  illustrations  of  them.  The 
solar  attachment  has  three  scales :  The  hour  circle,  the  declina- 
tion arc,  and  the  latitude  arc.  In  the  Burt  Solar  attachment, 
while  the  attachment  itself  does  not  contain  a  latitude  arc,  the 
vertical  limb  of  the  transit  is  utilized  as  such.  Vertical  limbs 
which  are  made  especially  to  be  used  in  conjunction  with  the 
Burt  Attachment  have  two  rows  of  numbers :  the  lower  set 
which  are  for  vertical  angle  measurements  represents  the  co- 
latitude  of  the  instrument  station ;  the  upper,  the  latitude 
(90°  minus  latitude  equals  co-latitude).  The  theory  of  the 
Attachment  depends  upon  the  facts  that  when  the  Latitude 
Arc  is  set  to  read  the  co-latitude  (or  latitude,  depending  upon 
the  vertical  scale  used)  ;  the  Declination  Arc  set  to  read  the 
declination  of  the  Sun  for  the  day  and  hour  corrected  for 
refraction ;  and  the  Hour  Circle  set  to  read  the  apparent 
time — then  the  ray  of  light  from  the  Sun  will  pass  along  the 
optical  axis  of  the  Attachment.  The  Burt  Attachment  has  a 
lens  and  silver  plate,  while  the  Telescopic  Attachment  has  a 
lens  and  a  prism,  the  silver  plate  or  prism  acting  as  a  screen  to 
catch  the  image  of  the  sun  formed  by  the  lens.  This  silver 
plate  and  prism  contain  two  vertical  parallel  lines  and  two 
horizontal  lines  which  intersect  and  form  a  square  which  is  just 
large  enough  to  include  a  circular  image  of  the  Sun.  When 
the  circular  image  coincides  with  the  four  sides  of  this  square 
at  the  same  instant,  then  the  ray  of  light  from  the  Sun  and  the 
optical  axis  of  the  Attachment  exactly  coincide.  When  the  sun 
is  north  of  the  equator,  the  Declination  Arc  of  Attachment  is 
towards  the  Sun,  and  when  the  Sun  is  south  of  the  equator,  the 
Declination  Arc  points  away  from  the  Sun.  For  these  reasons, 
each  end  of  the  Burt  Attachment  is  furnished  with  a  lens  and 
silver  plate,  in  order  that  the  Attachment  can  be  used  either 
way. 

*For    a    more    complete    description    of    the    solar    attachment    and    its    uses,    see 
Gurley's  Manual,  published  by  W.  &  L.  E.  Gurley. 


SURVEYOR'S   TRANSIT  WITH   BURT  SOLAR  ATTACHMENT 

Courtesy  of  W.  &  L.   E.  Gurley 


258 


Military  Topography  and  Photography 


Under  the  above  conditions,  having  one  condition  unknown, 
it  may  be  determined  by  making  the  other  adjustments,  and 
adjusting  the  unknown  condition  until  the  ray  of  light  from  the 
Sun  and  the  optical  axis  of  the  Solar  Attachment  coincide. 


*LIGHT  MOUNTAIN  TRANSIT  WITH  TELESCOPIC  SOLAR  ATTACHMENT 


"When  the  telescope  is  set  horizontal  by  its  spirit  level,  the 
hour  circle  will  be  in  the  plane  of  the  horizon,  the  polar  axis 
will  point  to  the  zenith,  and  the  zeros  of  the  vertical  arc  and  its 
vernier  will  coincide.  If  we  incline  the  telescope,  directed  north, 
the  polar  axis  will  descend  from  the  direction  of  the  zenith. 
The  angle  through  which  it  moves  [when  the  polar  axis  is 
directed  on  the  north  pole  of  the  heavens],  being  laid  off  on  the 
vertical  arc,  will  be  the  co-latitude  of  the  place  where  the  instru- 
ment is  used,  the  latitude  itself  being  found  by  subtracting  this 
number  from  ninety  degrees."! 

To  FIND  THE  LATITUDE.  "Level  the  instrument  very  care- 
fully, using  the  level  of  the  telescope,  until  the  bubble  will 


*Courtesy  of  W.  &  L.  E.  Gurley. 

t  Page  97,  Gurley's  Manual.     Courtesy  of  W.  &f  L.  E.  Gurley. 


Military  Topography  arid  Photography  259 

remain  in  the  middle  during  a  complete  revolution  of  the  instru- 
ment, the  tangent  movement  of  the  telescope  being  used  in  con- 
nection with  the  leveling  screws,  and  the  axis  of  the  telescope 
being  firmly  clamped. 

"Clamp  the  vertical  arc,  so  that  its  zero  and  the  zero  of  its 
vernier  coincide  as  near  as  may  be,  and  bring  them  into  exact 
line  by  the  tangent  screw  of  the  vernier. 

"Set  off  upon  the  proper  arc  the  declination  of  the  sun  for 
noon  of  the  given  day,  corrected  for  the  meridional  refraction, 
note  the  equation  of  time,  and  fifteen  or  twenty  minutes  before 
noon  direct  the  telescope  to  J;he  north  and  lower  the  objective 


*SEXTANT 

end  until  the  sun's  image  can  be  brought  nearly  into  position 
between  the  equatorial  lines  [the  two  horizontal  lines  on  the 
silver  plate  or  prism],  by  moving  the  instrument  upon  it,  its 
spindle  and  the  declination  from  side  to  side. 

"The  declination  arc  being  brought  directly  in  line  with  the 
telescope,  clamp  the  axis,  and  with  the  tangent  screw  of  the 
telescope  axis  bring  the  image  precisely  between  the  lines,  fol- 
lowing the  sun's  motion  as  the  image  runs  below  the  lower 
equatorial  line,  or,  in  other  words,  as  long  as  the  sun  continues 
to  rise  in  the  heavens. 

"When  the  sun  reaches  the  meridian  the  image  will  remain 
stationary  in  altitude  for  an  instant,  and  will  then  begin  to 
rise  on  the  plate. 


'Courtesy  of  W.  &  L.  E.   Gurley. 


260  Military  Topography  and  Photography 

"The  moment  the  image  ceases  to  run  below  is  apparent  noon, 
when  the  index  of  the  hour  arc  should  indicate  XII,  and  the 
latitude  be  determined  by  the  reading  of  the  vertical  arc."* 

To  FIND  THE  LONGITUDE.  In  order  to  use  the  Solar  Attach- 
ment for  determining  longitude,  it  is  necessary  to  have  some 
standard  meridian  time.  A  good  watch  adjusted  to  all  positions 
and  climatic  conditions  carrying  standard  time  will  do.  The 
Naval  observatory  time  is  sent  out  from  Washington  over  the 
Western  Union  lines  at  12  M.  Eastern  time:  it  reaches  all  points 
in  the  Central  Time  belt  at  11 :00  o'clock ;  in  the  Mountain  Time 
belt  at  10:00  o'clock;  and  in  the  Pacific  Time  belt  at  9:00 
o'clock  A.  M.  Standard  (Solar)  Time,  by  which  the  watch  can 
be  daily  checked  at  a  Western  Union  office.  Should  this  be 
impracticable,  as  where  a  party  is  in  the  field,  I  would  recom- 
mend the  following  procedure :  Select  an  easily  recognized  star 
that  transits  the  meridian  near  the  zenith,  and  two  solid  objects 
that  are  at  least  50  yards  apart  and  that  are  in  line  with  the 
selected  star  when  it  sets.  Those  objects  may  be  a  fence  post 
and  cone  of  a  roof,  or  a  distant  sharp  ridge.  As  soon  as  may 
be  observe  the  time,  which  is  perceptible,  that  the  selected  star 
falls  below  the  line  of  sight  determined  by  the  two  selected 
objects;  record  the  date  and  the  time  in  seconds.  This  star 
will  set  below  this  line  of  sight  approximately  3'56"  earlier 
every  night,  and  the  watch  can  be  daily  checked  by  it.  Thus, 
should  the  time  of  setting  have  been  8.13'22"  p.  M.  the  first  time, 
ten  days  later  it  will  set  39'20"  earlier,  or  at  7.34/2"  p.  M. 

To  find  the  longitude  by  means  of  the  Solar  Attachment,  the 
same  procedure  is  followed  as  in  finding  the  latitude,  as  ex- 
plained above,  except  that  at  "the  moment  the  image  ceases  to 
run  below,"  which  is  apparent  noon  at  the  place  of  observation, 
the  time  is  recorded  to  seconds.  The  difference  between  this 
recorded  time  and  XII  o'clock,  corrected  for  the  equation  of 
time  for  the  day  of  observation  as  shown  in  the  Solar  Ephcmeris 
tables,  is  the  true  difference  in  time  between  the  place  of  obser- 
vation and  the  meridian  of  reference.  This  difference  in  time 
can  be  changed  to  difference  in  longitude  (one  hour  equals  15° 
of  longitude),  from  which  the  longitude  of  the  place  of  obser- 
vation is  easily  computed. 

*Page  112,  Gurley's  Manual.     Courtesy  of  W.  &  L.  E.  Gurley. 


Military  Topography  and  Photography  261 

To  FIND  THE  TRUE  AZIMUTH.  When  the  latitude  is  deter- 
mined as  explained  above,  at  the  moment  of  apparent  noon,  the 
telescope  will  point  either  towards  the  true  north  or  true  south, 
depending  upon  whether  the  sun  is  north  or  south  of  the  equa- 
tor. Therefore,  if  the  A-vernier  is  set  to  read  180°  when  the 
sun  is  north  of  the  equator,  and  0°  when  the  sun  is  south,  and 
at  the  moment  of  apparent  noon,  as  determined,  the  lower 
horizontal  limb  of  the  transit  is  clamped,  then  the  transit  is 
exactly  oriented  in  true  azimuth. 

DETERMINATIONS  or  LATITUDE,  LONGITUDE,  AND  AZIMUTH  AT 
OTHER  TIMES  THAN  AT  APPARENT  NOON.  The  same  procedure 
is  followed  as  above  explained,  except  the  following:  The  hour 
circle  is  set  to  read  the  apparent  time  at  which  the  observation 
is  to  be  made  (this  circle  reads  to  five  minutes;  the  declination 
is  set  for  the  declination  at  the  hour  of  observation  of  the  day 
corrected  for  the  refraction  for  that  time :  the  A-vernier  is  set 
to  read  180°  when  the  sun  is  north  of  the  equator,  and  0°  when 
south :  the  lower  limb  is  undamped  and  the  transit  oriented  by 
compass  as  near  as  may  be,  it  is  then  clamped  and  the  image  of 
the  sun  is  brought  into  the  square  of  the  silver  plate  by  means 
of  the  lower  limb  tangent  screw  and  the  vertical  tangent  screw. 
At  the  moment  the  image  of  the  sun  is  brought  into  the  square, 
the  transit  will  be  oriented  in  true  azimuth  and  the  vertical  scale 
will  read  the  co-latitude  of  the  place  of  observation.  If  either 
the  true  azimuth  or  latitude  be  known  the  procedure  is  much 
simplified,  for  the  transit  can  be  at  once  oriented  in  true  azi- 
muth, or  the  co-latitude  set  on  the  vertical  scale,  requiring  the 
motion  of  only  one  screw  of  the  transit  to  bring  the  image  into 
the  square  on  the  silver  plate  of  the  Solar  Attachment. 

The  declination  of  the  sun  for  Greenwich  or  Washington 
apparent  noon  only  for  each  day  is  given  in  the  Ephemeris 
Tables.  The  hourly  difference  in  declination,  however,  for  each 
day  is  also  given,  from  which  the  declination  for  other  hour* 
than  at  apparent  noon  can  be  computed. 

*To  COMPUTE  THE  DECLINATION.  "Suppose  the  corrected 
declination  is  desired  for  the  different  hours  of  October  15,  1912, 
at  Troy,  N.  Y.  The  latitude  is  42°  44'.  The  longitude  is 


*Gurley'*  Manual. 


262  Military  Topography  and  Photography 

practically  five  hours:  so  that  the  declination  given  in  the 
Ephemeris  for  apparent  noon  of  that  day  at  Greenwich  would 
be  that  for  7  A.  M.  at  Troy,  or  five  hours  earlier.  Note  care- 
fully the  algebraic  signs.  The  declination  is  south  or  minus. 
Its  hourly  difference  is  minus.  The  refraction  always  plus. 
Hence  we  use  the  algebraic  sum,  thus : 
S  8°28'56".5  is  the  tabular  declination  for  7.00  A.  M. 

55".6  =  hr.  diff. 
8°29'52".l  +  ref.  (4  hrs.)  2'29"  =     -  8°27'23",    8.00  A.  M. 

55".6 
8°30'47".7  +  ref.  (3  hrs.)  1'39"  =     -  8°29'09",    9.00  A.  M. 

55".6 
8°31'43".3  +  ref.  (2  hrs.)  1'19"         -  8°30'24",  10.00  A.  M. 

55".6 
8°32'38".9  +  ref.  (1  hr.)    1'07"  =       -  8°31'32",  11.00  A.  M. 

55".  6 
8°33'34".5  +  ref.  (0  hr.)    1'07"  •=      -  8°32'27",  12       M. 

55".6 
8°34'30".l  +  ref.  (1  hr.)    1'07"  =      -  8°33'23",    1.00  P.  M. 

55".6 
8°35/25//.7  +  ref.  (2  hrs.)  1'19"  =      -  8°34'07",    2.00  P.  M. 

55".6 
8°36'21".3  +  ref.  (3  hrs.)  1'39"  =    -  8°34'42",    3.00  P.  M. 

55".6 

8°37'16".9  +  ref.  (4  hrs.)  2'29"  =  -  8°34'48",  4.00  P.  M. 
To  know  whether  to  add  or  subtract  the  hourly  difference  in 
declination  it  is  only  necessary  to  observe  whether  the  daily 
declination  is  increasing  or  decreasing.  The  abbreviation  "ref" 
above  means  "refraction."  A  table  can  be  made  in  which  the 
columns  represent  the  hours — VIII,  IX,  X,  XI,  XII,  I,  II, 
III,  IV ;  and  the  lines,  the  days  of  the  months  ;  and  the  declina- 
tion for  each  hour  placed  in  the  proper  square.  These  tables, 
a  complete  month  on  each  sheet,  can  be  mimeographed  or 
printed  in  the  office  and  furnished  to  the  field  parties.  The 
Ephemeris  Table  includes  the  following  for  each  day  of  the 
year :  the  date,  the  Sun's  apparent  declination,  the  hourly  dif- 
ference in  declination,  the  equation  of  time  to  be  added  or  sub- 
tracted from  the  apparent  time,  and  the  refraction  correction 


Military  Topography  and  Photography  263 

for  Lat.  40°  for  each  hour  of  the  day  time.  The  refraction 
correction  for  the  other  latitudes  is  found  by  multiplying  its 
latitude  coefficient  by  the  refraction  correction  for  40°.  The 
Ephemeris  Table  also  includes  a  table  of  latitude  coefficients. 

ACCURACY  AND  USE  OF  THE  SOLAR  ATTACHMENT.  Latitude 
and  longitude  determined  at  the  apparent  noon  are,  of  course, 
the  more  accurate.  By  using  a  magnifier  to  observe  the  sun's 
image  an  error  of  one  quarter  of  a  minute  in  azimuth  or  latitude 
has  been  detected.*  This  for  Lat.  40°  gives  the  true  latitude 
within  about  1500  feet;  and  a  like  error  for  longitude,  within 
about  1100  feet.  Thesennaximum  limits  of  errors  are  too  great 
for  geodetic  work,  but  are  sufficient  for  reconnaissance  and 
exploratory  surveys. 

Where  the  topographer  uses  a  transit  equipped  with  the  Solar 
attachment  and  a  sketching  board,  and  is  furnished  with  a  table 
of  computed  declinations,  he  can,  with  a  little  practice,  more 
quickly  and  accurately  determine  true  azimuths  for  the  orien- 
tation of  his  transit,  than  orientating  by  compass  aided  by 
"three-point  solutions";  the  oscillations  of  the  compass  and 
trials  to  reduce  the  triangle  of  error  to  a  point,  always  con- 
suming considerable  time.  Azimuths  determined  by  the  solar 
attachment  any  hour  of  the  day  are  sufficient  for  all  topo- 
graphical operations. 

f ADJUSTMENTS  OF  THE  SOLAR  ATTACHMENTS.  Solar  Lenses  and  Lines: 
"Detach  the  declination  arm  by  taking  off  the  clamp  and  tangent  screws, 
and  removing  the  center  by  which  the  arm  is  pivoted  on  the  arc. 

"Substitute  for  the  declination  arm  upon  the  attachment  the  adjusting 
bar  furnished  with  every  solar  instrument,  the  center  of  the  declination 
arm  fitting  into  the  hole  at  one  end  of  the  bar,  and  the  bar  being  further 
secured  to  the  attachment  by  the  clamp  screw  passing  through  the  hole  in 
the  declination  arc  left  by  the  removal  of  the  tangent  screw,  into  the 
threaded  hole  at  the  other  end  of  the  adjusting  bar,  thus  forming  a  support 
upon  which  the  declination  arm  can  be  adjusted. 

"Place  the  declination  arm  on  the  adjuster,  turn  one  end  to  the  sun, 
and  bring  it  into  such  a  position  that  the  image  of  the  sun  is  made  to  appear 
precisely  between  the  equatorial  lines  on  the  opposite  plate. 

"Carefully  turn  the  arm  over,  until  it  rests  upon  the  adjuster  by  the 
opposite  faces  of  the  rectangular  blocks,  and  again  observe  the  sun's  image. 
If  it  remains  between  the  lines  as  before,  the  arm  is  in  adjustment.  If  not, 

*Page   100,   Gurley's  Manual,  which  see. 

fPages  124-126,  Gvrley's  Manual.     Courtesy  of  W.  &  L.  E.  Gurley. 


264  Military  Topography  and  Photography 

loosen  the  three  small  screws  and  move  the  silver  plate  under  their  heads 
until  one  half  the  error  in  the  position  of  the  sun's  image  is  removed. 

"Bring  the  image  again  between  the  lines,  and  repeat  the  operation  as 
above  on  both  ends  of  the  arm,  until  the  image  will  remain  between  the 
lines  of  the  plate  in  both  positions  of  the  arm,  when  it  will  be  in  proper 
adjustment,  and  the  arm  may  be  placed  in  its  former  position  on  the  attach- 
ment. This  adjustment  is  very  rarely  needed  in  our  instruments,  the  lenses 
being  cemented  in  their  cells  and  the  plates  securely  fastened. 

"To  adjust  the  Vernier  of  the  Declination  Arc:  Set  the  vernier  at  zero, 
and  raise  or  lower  the  telescope  until  the  sun's  image  appears  exactly  be- 
tween the  equatorial  lines. 

"Having  the  telescope  axis  clamped,  carefully  revolve  the  arm  until  the 
image  appears  on  the  other  plate.  If  precisely  between  the  lines  the  adjust- 
ment is  complete.  If  not,  move  the  declination  arm  by  its  tangent  screw 
until  the  image  will  come  precisely  between  the  lines  on  the  two  opposite 
plates.  Clamp  the  arm  and  remove  the  index  error  by  loosening  two  screws 
that  fasten  the  vernier:  place  the  zeros  of  the  vernier  and  limb  in  exact 
coincidence,  tighten  the  screws  and  the  adjustment  is  complete. 

"To  Adjust  the  Polar  Axis:  Level  the  instrument  carefully  by  the 
long  level  of  the  telescope,  using  the  tangent  movement  of  the  telescope 
axis  in  connection  with  the  leveling  screws,  until  the  bubble  will  remain  in 
the  middle  during  a  complete  revolution  of  the  instrument  upon  its  axis. 

"Place  the  solar  attachment  upon  the  axis  and  see  that  it  moves  easily 
around  it.  Bring  the  declination  arm  into  the  same  vertical  plane  with  the 
telescope,  place  the  adjusting  level,  *  *  *  ,  upon  the  top  of  the  rec- 
tangular blocks,  and  bring  the  bubble  of  the  level  into  the  middle  by  the 
tangent  screw  of  the  declination  arc. 

"Turn  the  arc  half  way  around,  bringing  it  again  parallel  with  the  tele- 
scope, and  note  the  position  of  the  level.  If  in  the  middle,  the  polar  axis 
is  vertical  in  that  direction.  If  not  in  the  middle,  correct  one  half  the  error 
by  the  capstan  head  adjusting  screws  under  the  base  of  the  polar  axis, 
moving  each  screw  of  the  pair  the  same  amount,  but  in  an  opposite  direc- 
tion. Bring  the  level  to  the  middle  again  by  the  tangent  screw  of  the 
declination  arc,  and  repeat  the  operation  as  before,  until  the  bubble  will 
remain  in  the  middle  when  the  adjusting  level  is  reversed. 

"Pursue  the  same  course  in  adjusting  the  arc  in  the  second  position,  or 
over  the  telescope  axis,  and  when  completed  the  level  will  remain  in  the 
middle  during  an  entire  revolution  of  the  arc,  showing  that  the  polar  axis 
is  at  right  angles  with  the  level  under  the  telescope,  or  truly  vertical. 

"As  this  is  by  far  the  most  delicate  and  important  adjustment  of  the 
solar  attachment,  it  should  be  made  with  the  greatest  care,  the  bubble  being 
kept  precisely  in  the  middle  and  frequently  inspected  in  the  course  of  the 
adjustment. 

"The  adjusting  level  is  supposed  to  be  itself  in  adjustment:  but  if  not, 
it  can  be  easily  corrected  by  the  screw  shown  at  one  end,  when  reversed 
upon  a  plane  surface,  exactly  as  a  mason's  level  is  adjusted. 

"To  adjust  the  Hour  Arc:  Whenever  the  instrument  is  set  in  the 
meridian,  *  *,  the  index  of  the  hour  arc  should  read  apparent  time. 
If  not,  loosen  the  two  flat  head  screws  on  tfte  top  of  the  hour  circle,  and 


Military  Topography  and  Photography  265 

with  the  hand  turn  the  circle  around  until  the  proper  reading  is  indicated, 
fasten  the  screws  again,  and  the  adjustment  will  be  complete." 

*ADJUSTMEKTS  OF  THE  TELESCOPIC  SOLAR.  "1.  Unscrew  the  tangent 
screw  of  the  declination  arc  from  the  nut  and  remove  the  reflector  together 
with  its  axis,  by  unscrewing  the  caps  at  its  bearings. 

"2.  Adjust  the  line  of  collimation  by  revolving  the  telescope  in  its 
bearings,  using  as  distant  a  point  as  possible. 

"3.  Cause  the  telescope  to  trace  a  vertical  line  and  align  with  the  main 
telescope  by  adjustment  of  the  four  capstan  head  screws  of  the  axis  of  the 
frame. 

"4.  With  the  main  telescope  leveled,  its  line  and  that  of  the  solar  tele- 
scope should  agree:  if  not,  adjust  the  solar  telescope  by  moving  the  tangent 
screw  of  the  latitude  arc.  When  the  two  lines  are  in  coincidence,  the  latitude 
arc  should  read  0°.  +.. 

"5.  Replace  the  reflector  and  tangent  screw  of  the  declination  arc. 
Lay  off  two  points  ninety  degrees  apart,  one  should  be  in  good  illumina- 
tion, a  point  projecting  above  the  sky  line  is  to  be  preferred.  Set  the  imin 
telescope  on  one  point  and  get  the  reflected  image  of  the  other  point  through 
the  solar  telescope,  moving  it  by  means  of  the  tangent  screw  of  the  declina- 
tion arc.  The  declination  arc  should  then  read  0°. 

"6.  Lay  off  the  latitude  and  corrected  declination  on  their  respective 
arcs  and  bring  the  sun's  image  inscribed  in  the  cross  wires  by  revolving 
the  transit  about  its  vertical  axis,  and  the  solar  telescope  about  its  axis. 
The  instrument  is  now  on  the  meridian  from  which  any  angle  may  be  taken. 
There  is  no  further  change  except  the  hourly  change  of  decimation." 

MAP  REPRODUCTION  AND  ENLARGEMENT 

The  subject  of  Map  Reproduction  and  Enlargement  will  be 
outlined  here  only  in  a  general  way,  for  its  principles  have  been 
otherwise  sufficiently  discussed,  for  field  work  in  the  operations 
of  topographical  surveying  and  rapid  sketching. 

MAP  REPRODUCTION.  Tracing:  In  this  method  of  map 
reproduction,  a  sheet  of  transparent  tracing  paper  is  placed 
over  the  map  to  be  traced  and  the  lines  of  the  map  are  marked 
on  the  tracing  paper  "with  pen  or  pencil. 

Carbon  Tracing:  In  this  method  a  sheet  of  paper  is  placed 
under  the  map  with  a  sheet  of  carbon  paper  between,  and  the 
lines  of  the  map  directly  traced  with  pencil  or  graphic  pen. 
The  under  sheet  of  paper  must  be  so  attached  to  the  map  that 
there  can  be  no  slipping  out  of  position. 

Free  Hand:  In  this  method  the  map  is  divided  into  blocks 
by  drawing  north  and  south,  and  east  and  west  lines,  one  or 
two,  or  more  inches  apart ;  the  sheet  of  paper  upon  which  the 

*Page  117,  Gurley's  Manual.     Courtesy  of  W.  &  L.  E.  Gurley. 


266  Military  Topography  and  Photography 

reproduction  is  to  be  made  is  similarly  divided.  By  the  use 
of  dividers  sufficient  control  points  within  each  block  on  the 
map  may  be  determined  and  plotted  on  the  blank  sheet,  so  that 
all  the  lines  within  a  block  can  be  sketched  in  free  hand. 

Blue  Printing:  All  are  more  or  less  familiar  with  this  pro- 
cess. Blue  print  paper  can  be  purchased  either  in  sensitive  or 
unsensitive  form.  The  former  is  kept  in  light-tight  rolls,  and 
several  rolls  in  a  tin  case.  To  sensitize  paper  in  the  field  the 

following  solutions  are  used: 

Solution  A:     Citrate  of  Iron  and  Ammonia  2  ozs. 

Water  8  ozs. 

Solution  B:     Red  Prussiate  of  Potash  2  ozs. 

Water  8  ozs. 

For  immediate  use  mix  4  parts  of  "A"  with  3  parts  of  "B." 
A  sheet  of  paper  of  the  desired  size  is  cut  from  the  roll  and  laid 
on  a  flat  surface ;  the  mixed  solution  is  applied  with  a  sponge, 
care  being  taken  not  to  wet  the  paper  clear  through ;  the  sheet 
is  then  hung  up  in  a  dark  room  to  dry,  after  which  it  is  ready 
for  use. 

The  process  of  printing  blue  paper  is  much  similar  to  print- 
ing "printing  out"  papers  in  photography.  A  plain  glass  is 
used  in  the  printing  frame  against  which  a  sheet  of  tracing 
linen  containing  the  map  is  held  in  place,  the  tracing  linen 
forming  the  negative ;  a  sheet  of  sensitive  blue  paper  is  then 
inserted  in  the  printing  frame  with  the  sensitive  side  next  to 
the  tracing  linen ;  a  board  of  the  same  size  as  the  frame  holds 
the  tracing  linen  and  blue  paper  tightly  against  the  glass  of  the 
printing  frame.  The  frame  with  the  linen  and  blue  paper  is 
then  exposed  to  the  direct  sunlight  for  from  four  to  eight  min- 
utes, after  which  the  blue-print  paper  is  taken  out  of  the  frame 
and  placed  in  clear  water  sufficient  to  cover  the  whole  surface 
of  the  print.  It  should  be  rinsed  until  the  lines  stand  out  in 
clear  white  and  then  hung  up  to  dry.  The  lines  of  the  map  on 
the  tracing  linen  should  of  course  have  been  drawn  well  defined. 

Additions  and  alterations  may  be  made  by  using  a  10% 
solution  of  Oxalate  of  Potash  as  an  ink,  adding  a  little  mucilage 
if  the  solution  tends  to  run. 

White  Printing:  By  using  either  a  blue  or  brown  print  as 
a  negative,  a  positive  print  of  white  background  and  black 


Military  Topography  and  Photography  267 

lines  may  be  obtained.  In  such  cases  the  blue  or  brown-print 
used  must  have  especially  well-defined  lines  developed  on  it. 

Photographic  Method:  This  method  is  seldom  used.  It  is 
very  useful,  however,  where  a  small  photograph  is  desired  of 
some  large  map,  and  especially  where  such  large  maps  are  in 
considerable  numbers,  and  it  is  desired  to  transport  copies  to 
a  distance.  The  map  is  placed  on  a  wall  and  a  picture  taken 
of  it ;  the  only  special  precautions  necessary  are  that  the  map 
be  well  stretched  and  perfectly  flat  on  the  wall. 

Lithographic  Methods:  Map  reproduction,  or  printing, 
in  the  field  is  generally  limited  to  the  blue  print  method.  In 
time  of  peace  where  a  large  number  of  military  maps  are  desired, 
the  lithographic  methods  should  be  employed.  The  printing 
is  done  lithographically  from  either  stone  or  metal  plates. 
Stone  and  metal  plates  can  be  most  easily  made  by  photolitho- 
graphic methods,  in  which  either  sensitized  aluminum  or  zinc 
plates  are  printed  on  photographically  through  a  negative  of 
the  map,  or  a  photo-lithographic  print  of  the  map  is  made  on 
transfer  paper  which  is  printed  lithographically  on  stone  or 
metal  plates.  The  best  map  printing,  however,  is  done  by  en- 
graving the  map  on  a  copper  plate,  transferring  the  same  from 
the  plate  to  a  stone  lithographically,  and  printing  from  the 
stone. 

MAP  ENLARGEMENT.  By  Pantagraph:  The  pantagraph  is 
a  common  and  well-known  instrument  to  most  people.  It  may 
be  used  to  reduce,  enlarge,  or  reproduce  a  map,  but  only  one 
copy  can  be  produced  at  a  time. 

Photographic  Method:  The  camera  may  also  be  used  to 
enlarge  small  maps.  The  map  is  first  photographed  on  a  dry 
plate,  and  then  by  allowing  light  to  pass  through  the  negative 
while  in  the  camera  (the  slides  and  partition  of  a  plate  holder 
having  been  removed)  an  image  is  formed  in  front  of  the  camera 
at  the  desired  distance  away  which  sensitizes  a  sheet  of  printing 
paper  placed  at  that  position.  The  arrangement  of  the  ap- 
paratus is  as  shown  in  the  diagram,  Fig.  104,  which  explains 
itself;  the  operation  is  carried  out  in  a  dark  room.  M  is  a 
mirror  placed  at  an  angle  of  45°  to  reflect  the  light  uniformly 


268 


Military  Topography  and  Photography 


FIG  104 


N! 

ti 


through  the  negative  which  is  held  in  the  camera  C;  S  is  the 
screen  upon  which  the  printing  paper  is  placed — it  may  be 
moved  forward  or  backward  to  secure  a  map  of  the  desired  size. 

By  Coordinates:  The  map  is  divided  into  blocks  by  north- 
south  lines  and  east-west  lines,  generally  the  same  distance 
apart.  Similar  blocks  are  drawn  on  a  blank  sheet  of  drawing 
paper  with  the  ratio  between  the  lines  and  the  corresponding 
ones  on  the  map  the  same  as  the  desired  increase  in  size.  By 
the  aid  of  these  lines  the  horizontal  position  of  a  sufficient  num- 
ber of  controlling  points  of  the  map  are  estimated  on  the  blank 
sheet  to  enable  the  map  to  be  plotted  free  hand. 

POLYCONIC  PROJECTIONS.  The  polyconic  projection  shown 
in  Fig.  105,  was  drawn  in  the  following  manner:  A  vertical 
line  AA'  is  drawn  across  the  center  of  a  sheet.  This  forms  the 
central  meridian,  and  it  is  divided  into  the  divisions  mlm29  m2m3, 
m3m4,  which  are  equal  to  the  difference  in  latitude  between  the 
parallels  shown.  These  differences  in  meters  can  be  found  from 


Military  Topography  and  Photography  269 

column  3  of  TABLE  V.  For  example,  if  every  degree  of  lati- 
tude were  being  shown  as  a  parallel,  the  amount  as  found  in 
Col.  3,  Table  V,  would  have  to  be  multiplied  by  3600;  for  the 
amount  given  there  is  for  1"  of  latitude. 

A  line  BB'  is  then  drawn  through  point  nil  perpendicular  to 
AA',  and  similar  lines  through  m2,  m3,  and  m4.  On  these  lines 
are  laid  off  the  distances — plotted  to  scale — nuni,  m2n2,  m3n3, 
and  m4n4  from  the  central  meridian,  equal  respectively  to  the 
values  of  longitudes  for  the  respective  parallel  of  latitude  on 
which  the  distances  are  laid  off,  and  which  are  found  from 
Column  4  of  Table  V  (column  headed  "X").  Through  these 
points,  ni,  n2,  n3,  and  n4,  are  drawn  curved  lines,  which  lines  are 
the  other  meridians  of  the  sheet.  At  points  HI,  n2,  n3,  and  n4, 
distances  equal  to  YI,  Y2,  Y3,  and  Y4,  as  found  in  Column  5, 
Table  V  (column  headed  "Y")  are  laid  off  along  the  meridians, 
and  through  these  points  and  the  points  mi,  m2,  m3,  and  m4  are 
drawn  curved  lines  to  represent  the  parallels  of  latitude. 


CONVENTIONAL  SIGNS 
UNITED  STATES  ARMY 
MAPS 

Courtesy  of  United  States  Geological  Survey 
REPRINTED  BY  PERMISSION  OF  THE  SECRETARY  OF  WAR 


272 


Military  Topography  and  Photography 


WORKS  AND  STRUCTURES 


Canal  or  Ditch  «, 


Aqueduct  or  Waterpipe., 

Aqueduct  Tunnel „ ^^-.,.r^ 

Canal  Lock  (point  up  stream) „ „ 1 

Metaled ....=_====== 

Good ^— 

Wagon  Roads 

Poor  or  Private  „ 

On  small-scale  maps 

Trail  or  Path .... 

Railroad  of  any  kind 

(or  Single  Track) 

Double  Track  ...„ 

Juxtaposition  of , 

Railroads 

Electric / "~ 

In  Wagon  Road  or  Street          Steam          Electric 

Tunnel ,. .„ _>_ 

Railroad  Station  of  any  kind........ ^ 

Symbol  (modified  below)  T 

Along  road „ ^ 

Telegraph  Line     1 

Along  road 

(small-scale  maps) 

Along  trail 

V 
Electric  Power  Transmission  Line 


Military  Topography  and  Photography  273 


WORKS  AND  STRUCTURES 


Bridges    < 


General  Symbol 

*.- 

Drawbridges  (on  large-scale      I 
charts  leave  channel  open)       \ 

Truss  (W,  Wood;  S,  Steel) 


Suspension 

Arch 

Pontoon 

Ferries _ 

(General  Symbol 
(or  Wagon  and  Artillery) 
Infantry  and  Cavalry 
Cavalry...,. 

Dam 


274  Military  Topography  and  Photography 


WORKS  AND  STRUCTURES 


Buildings  in  general  „..., _ 

Ruins ,™ „ „... „ 

Church „_ 

Hospital 

Schoolhouse 

Post  Office : ...-, 

Telegraph  Office 


e 


*  or  * 


lor  .  SM 


Waterworks .-. , 

Windmill 

City,  Town,  or  Village 

City,  Town,  or  Village  (generalized) 


City,  Town,  or  Village 

(small- scale  maps) 


Capital 

County  Seat 
Other  Towns 


&07-* 


o 


Military  Topography  and  Photography  275 


WORKS  AND  STRUCTURES 


Cemetery \.c.*"..\  or  it  • 

ftr 

Mine  or  Quarry  of  any  kind  (or  open  cut) * 

Prospect „ _ x 

Shaft e 

Mine  Tunnel J0penin*  ~  A 

(Showing  direction >. ^          ^  r<~^ 

Oil  Wells :. Oooo  0 

•      •  9  ,     ^    ^ 
On  Tanks  (abbreviation  OT) . ;.  •  • 

CoA-e  Ovens „ „.„ „ 

Fence  of  any  kind 

(or  board  fence) 

Stone » .pq^^-r-r^^p, 

Fences <  Worm „ 

Win _ 

_ -_ - ~ K  «•»  O  «:>  -if  if  CC«»  ';> 


276  Military  Topography  and  Photography 


BOUNDARIES,  MARKS.  AND  MONUMENTS 


National,  State,  or  Province  Line  __  -  -.  -- 

County  Line 

Civil  Township,  District, 

Precinct,  or  Barrio 

Reservation  Line 

Land-Grant  Line 

City,  Village,  or  Borough 

Cemetery,  Small  Park,  etc. 


Township,  Section,  and  Quarter  Section  ( 
Lines  (any  one  for  township  line  alone,  any  \  — 
two  for  township  and  section  lines  ... 


Township  and  Section  Corners  Recovered—  +  __  .j.  _____  i... 

Boundary  Monument  .................  ................  ..................  _  ^  _  __ 

Triangulation  Station 


Bench  -mark  8XM 

1232 

U.  S.  Mineral  Monument ...... 


Military  Topography  and  Photography  277 


DRAINAGE 


Streams  in  general 


Intermittent  Streams 


Lake  or  Pond  in  generaf' 

(with  or  without  tint,  waterlining,  etc.) 


Salt  Pond  (broken  shoreline  if  intermittent) 


Intermittent  Lake  or  Pond  ..... 


Spring 

Falls  and  Rapids  ........ 


Glaciers 


Contours 

(or  as  below) 


Form  Lines  showing  flow 


278  Military  Topography  and  Photography 


RELIEF 

(Shown  by  cdntours,  form  lines,  or  shading  as  desired) 

Hill  Shapes- 


Form  lines,  hachures, 

,    stipple, 
or  other  shading 


Contour  System - 


Depression  Contours,  if  otherwise 
Ambiguous,  haohured  thus 


crib 


Bluffs 


Rocky  (or  use  contours) 


\pther  than  rocky  (or  use  contours) 


Sand  Dunes 


Levee 


ftf§ 


Military  Topography  and  Photography 


279 


LAND  CLASSIFICATION 


/ftfars/i  in  general  (or  Fresh  Marsh) ^==^--^ 


Marsh  < 


Salt 


Wooded 


[Cypress  Swamp 


Woods  of  any  kind  (or  as  shown  oe/ow) 


Woods  Of  any  kind  (or  Broad-Leaved  Trees) 


280  Military  Topography  and  Photography 


LAND  CLASSIFICATION 


Pine  (or  //«rrow-L»«v«</  Trees) 


*'*»****  &.*  v"«  f 

£•?**•  JV?  *»*i* 

^4d*  **  J*  \^+-*_     -<x.     ^ 


Pteto 


Palmetto 


Mangrove 


Bamboo.. 


•^^         t'1'"''**      **^ft    +* 

t^+ ;?;  ;^  /* 
*  *t*  ^  *;  t^. 


Military  Topography  and  Photography  281 


LAND  CLASSIFICATION 


Cactus..^ 


Banana 


Orchard 


Grassland  in  general 


Tall  Tropical  Grass _.... 


O 


»!(//    "     •»'"•''"'         "'   „,„.     •"' 
" 


282  Military  Topography  and  Photography 


LAND  CLASSIFICATION 


Cultivated  Fields  in  general 


Cotton. 


Sugar  Cane. 


*     •      ._     rt     •         >,'€>'>*"    O        f\'fi 

^  O  o/°.  o-  o    .  o  •  •    o 
°W  „.-*  °  ^  •  o  o  'o      o- 

( '   O.O .    ...•  O  .     •   ,    •        ov'  ra 


Rice 


A   ^    rj,    ^    t    r( 


Corn. 


Military  Topography  and  Photography  2£<3 


HYDROGRAPHY.  DANGERS   OBSTRUCTIONS 


Shorelines 


( Surveyed 

I  Unsurveyed  .... 


/ Tidal  Flats  of  any  kind 
(or  as  shown  below) 


Shores  and 
Low-Water  Lines 


Rocky  Ledges 


Gravel  and  Rocks 


Mud 


284  Military  Topography  and  Photography 


HYDROGRAPHY,  DANGERS,  OBSTRUCTIONS 


Coral  Reefs 

Kelp _.... 

Eel  Grass 

Rock  under  water [ _ 

Rock  awash  (at  any  stage  of  the  tide) 

Rock  whose  position  is  doubtful  ..„.._ 

Rock  whose  vxistence  is  doubtful  ... 

Overfalls  and  Tide  Rips ..... 

Limiting  Danger  Line 

Whirlpools  and  Eddies 


Wreck  Of  any  kind  (or  Submerged  Derelict)   _ _ ...^. 

Wreck  or  Derelict  not  submerged ju. 

Cable  (with  or  without  lettering) 


Military  Topography  and  Photography 


285 


HYDROGRAPHY,  DANGERS,  OBSTRUCTIONS 


Current,  not  tidal,  velocity  2  knots 


Tidal  Currents 


(Flood,  ftknots 

Ebb,  1  knot 

Flood,  2d  hour — 


or 


\  Ebb,  Zd  hour __~^ 

No  bottom  at  50  Fathoms „ 

Abbreviations  relating  to  Bottoms 


s 


M.  mud,  S.  sand,  G.  gravel,  Sh.  Shells,  P.  pebbles,  Sp.  specks, 
Cl.  clay,  St.  stones,  Co.  coral,  Oz.  ooze,  bk.  black,  wh.  white,  rd.  red, 
yl.  yellow,  gy.  gray,  bu.  blue,  dk.  dark.  It.  light,  gn.  green,  br.  brown 
hrd.  hard,  sft.  soft,  fne.  fine,  crs.  coarse,  rky.  rocky,  stk.  sticky, 
brk.  broken.  Irg.  large,  sml  small,  stf.  stiff,  caL  calcareous,  dec. 
decayed,  rot.  rotten,  spk.  speckled,  fly.  flinty,  gty.  gritty,  grd.  ground, 
str.  streaky,  vol.  volcanic. 


286  Military  Topography  and  Photography 


HYDROGRAPHY,  DANGERS.  OBSTRUCTIONS 

Depth  Curves 
1  Fathom  or  6  Foot  Line 


2  Fathom  or  12  Foot  Line 

3  Fathom  or  18  Foot  Line _ 

4  Fathom  Line 

4%  Fathom  Line - 

5  Fathom  Line 

6  Fathom  Line 

10  Fathom  Line - 

20  Fathom  Line 

SO  Fathom  Line 

40  Fathom  Line  

50  Fathom  Line 

100  Fathom  Line 

200  Fathom  Line 

I 

300  Fathom  Line 

500  Fathom  Line . 

JOOO  Fathom  Line 

2000  Fathom  Line    _ 

5000  Fathom  Line 


Military  Topography  and  Photography  287 


AIDS  TO  NAVIGATION.  ETC. 

Life-saving  Station  ..........................................................  +t.s.s.  (T) 

[(T)  indicates  telegraphic  connection] 

Light  of  any  kind  (or  Lighthouse)  ...........................................................  41 

Lighthouse,  on  small  scale  chart  .....................................................................................  • 

Light  Vessel  of  any  kind  ....................................................  .* 

Light  Vessels  showing  number  of  masts  ................................. 

Light  with  Wireless 

Light  Vessel  with  Wireless  ........................ 


Light  with  Submarine  Bell  .......................................................................................  &    j 

Light  Vessel  with  Submarine  Bell  ...............................................................     ^ 

Light  with  Submarine  Bell  and  Wireless  .........  .  ................  .  ........  flfo   g 

Light  Vessel  with  Submarine  Bell  and  Wireless  .........  ...    (g 

/  Lighted   ..................  .  ..........................................................................  -  ..................    fc    * 

Beacons 

{  Not  lighted.  ......1...  ....................................................  8n*  1  i  1  I  I  1 

Sectors,  shown  by  dotted  lines 

Abbreviations  relating  to  Lights 

F.  fixed,  Fig.  flashing,  Fl.  flash,  Fls.  flashes,  Sec.  sector,  Rev.  revolv- 
ing, E.  electric,  W.  white,  R.  red,  V.  varied  by.  Orp.  group,  Occ. 
occulting,  Int.  intermittent,  Alt.  alternating,  m.  miles,  min.  minutes, 
sec.  seconds. 


288 


Military  Topography  and  Photography 


Buoys 


AIDS  TO  NAVIGATION,  ETC. 

Buoy  of  any  kind  (or  Red  Buoy) 

Black ,_ „...„.„ 

Striped  horizontally. __ 

Striped  vertically.— „ 

Checkered 

Perch  and  Square. _ .-. 

Perch  and  Ball ,-... 

Whistling  (or  use  first  four  symbols 

with  word  "  whistling  ") 

Bell  (or  use  first  four  symbols  with 

word  "bell") 

.Lighted 


Wt 

6i$$ 


Spindle  or  Stake  (add  word  "spindle" _ ...  I 

if  space  allows) 

Abbreviations  relating  to  Buoys 

C.  can,  N.  nun,  S.  spar,  H.  S.  horizontal  stripes,  B.  black,  R.  red, 
W.  white,  V.  S.  vertical  stripes,  G.  green,  Y.  yellow,  Ch.  checkered. 


Anchorage 


Of  any  kind  (or  for  large  vessels) 


,For  small  vessels 

Mooring  Buoy 

Range  or  Track  Line 


Military  Topography  and  Photography  289 


SPECIAL  MILITARY  SYMBOLS 

Regimental  Headquarters l»l 

2B 

Brigade  Headquarters i 4D*ac 

Division  Headquarters stffcac 

Corps  Headquarters r - y£ 

Infantry  in  line »~ c±3 

Cb 

Infantry  in  column _     g 

Cavalry  in  line dm 

A 

Cavalry  in  column a 

tm 
Mounted  Infantry «^= 

Artillery *  i|i 

Sentry ~ -.- - 

Vidette '. „... - 

Picket,  Cavalry  and  Infantry _ „. 

Support,  Cavalry  and  Infantry 

Wagon  Train..... -......* „ -at*  •«  • 

Adjutant  General..... _ ,.. „. 

Quartermaster _ «. 

Commissary ^..^. „ - , „ 


290  Military  Topography  and  Photography 


SPECIAL  MILITARY  SYMBOLS 

Medical  Corps 

Ordnance .... O 

Signal  Corps , _ -.r1 

Engineer  Corps : _ _ Ji£ 

Gun  Battery 

Mortar  Battery 

Fort         \ 

>  True  plan  to  be  shown  if  known 
Redoubt    ) 

A  A  A. 

Camp     _ 


Battle 
Trench 

When  color  is  used  execute  the  following  in  red 

Abattis^  "Y  ^J 

Wire  Entanglement 

Palisades^ __„ Illlllllillllll 

Contact  mines o  °  o  °o° 

Controlled  mines 
Demolitions 


Military  Topography  and  Photography  291 


LETTERING 

CIVIL     DIVISIONS 

States,  Counties,  Townships,  Capitals  a/id 
Principal   Cities   (aJL  capital  letters) 

ABCDEFGHIJ 

KLMNOPQRST 

UVWXYZ 

Towns   and   VHLag&s  (with  Cap.  initials) 
ab  cdefghijklmnopqr  stuvwxy  z 

HYDROGRAPHY 

Lakes,  JOyers   and  Bays   (aZL  capital  letters) 

ABCDEFGHIJ 

KLMNOPQRST 

UVWXYZ 

Creeks,  Brooks,  Springs.  sjrtaJl  JLafoes,  fbnds, 
Marsh&s  and  Glaciers  (wi&i  Cap.  initials ) 


292  Military  Topography  and  Photography 


LETTERING 

HYPSOGRAPHY 

,  Plateaus,  Lines   of  Clzffs 
and'  Canyons  (aJl  capital  letters) 

ABCDEFGHIJKLMNOPQRSTU 
VWXYZ 

Peaks,  small  Valleys,  Canyons,  Islands    and   Points. 

(wvttv  Cap.  initials) 
abcdefghijklmnopqrstuvwxyz 

PUBLIC     WORKS 

Railroads,  Tunnels,  bridges,  Ferries,  Wagon-Toads, 
Trails,  Fords   and  Dams  f  capitals  onfrj 

ABCDtFGHIJKLMNOPQPSTU  VWXYZ 

CONTOUR    NUMBERS 

123^567890 


MARGINAL     LETTERING 

AB  CDEFGHIJKLMNOPQRSTU 
VWXYZ 


Cop.  wttiolaj 

abcdefghijklmnopqrstuvwxyz 
J234567890 


Military  Topography  and  Photography 


293 


LETTERING 


Names  of  natural  land  features,  vertical  lettering 
Names  of  natural  water  features,  slanting  lettering 

Thickness  of  letter  f  of  height  A 

Slope  of  letter  3  parts  of  base  to  8  of  height 

AUTHORIZED  ABBREVIATIONS 


A. 

Arroyo 

L.S  S.Life  Saving  Station 

abut. 

Abutment 

L.H.      Lighthouse 

A 

Arch 

Long    Longitude 

b 

Brick 

Mt         Mountain 

B  s: 

Blacksmith  Shop 

Mts.      Mountains 

bot 

Bottom 

N           North 

Br. 

Branch 

nf       Notfordable 

br. 

Bridge 

p.          Pier 

C.  • 

Cape 

pk.        Plank 

cem 

Cemetery 

P.  O.      Post  Office 

con 

Concrete 

Pt.         Point 

cov. 

Covered 

qp.       Queen-post 

Cr 

Creek 

R.         River 

cui. 

Culvert 

R.H.     Roundhouse 

D.S 

Drug  Store 

R.R      Railroad 

E 

East 

S.          South 

Est. 

Estuary 

s.          Steel 

f. 

Fordable 

S  H.     School  House 

Ft. 

Fort 

S  M.     Saw  Mill 

G  S. 

General  Store 

Sta.      Station 

6ir. 

Girder 

si.        Stone 

G.M. 

Grist  Mill 

str.       Stream 

i. 

Iron 

T.G.      Toll  Gate 

1. 

Island 

Tres      Trestle 

Jc. 

Junction 

tr.         Truss 

**• 

L. 

King-post 
Lake 

W.T.     Water  Tank 
W  W.     Waterworks 

Lat. 

Latitude 

W.         West 

Ldg 

Landing 

w.          Wood 

294  Military  Topography  and  Photography 


DENSE  WOODS  OR  WOODS  OF  HEAVY  UNDERGROWTH.    SERIOUS  OBSTRUCTION 
TO  MOVEMENT  OF  TROOPS.     NECESSARY  TO  CUT  PASSAGE. 


Military  Topography  and  Photography  295 


TABLES 

PAGE 

I.     CONVERSIONS    298 

II.     TRIGONOMETRIC  FORMULA    299 

III.  LOGARITHMS  OF  NUMBERS 301 

IV.  STADIA  REDUCTIONS     303 

V.     POLYCONIC  PROJECTIONS                                                 .  305 


296  Military  Topography  and  Photography 

TABLE  I 

CONVERSIONS 

LINEAR  MEASURE — ENGLISH  —  METRIC 

1  Inch  =  —  2.54  Centimeters 

12  Inches  =  1  Foot  =  30.48  Centimeters 

3  Feet  =  1  Yard  —  .9144  Meters 

5i/2  Yards  =  1  Rod  —  4.572  Meters 

320  Rods  =  1  Mile  =  1.463  Kilometers 

LINEAR  MEASURE — METRIC  =  ENGLISH 

1  Centimeter  —  =  .3937  Inches 

100  Centimeters  =  1  Kilometer  =  3.28  Feet 

1000  Meters  =  1  Meter  —  1093.6  Yards 

SQUARE  MEASURE — .ENGLISH  =  METRIC 

1  Square  Rod  —  272.25  Sq.  Feet  =  20.9  Sq.  Meters 

1  Acre  ==  160  Sq.  -Rods  =  .405  Hectares 

1  Sq.  Mile  =  640  Acres  =  2.14  Sq.  Kilometers 

SQUARE  MEASURE — METRIC  =.  ENGLISH 

1  Sq.  Meter  =  10.76  Sq.  Feet 

10,000  Sq.  Meters  '=  1  Sq.  Kilometer  =  2.47  Acres 

100  Hectares  •=  1  Hectare  =  247  Acres 

WEIGHTS — ENGLISH 

Avoirdupois  =  1  Pound  =  16  Ounces  =  256  Drams  =  7,000  Grains 
Apothecaries  =  1  Pound  =.  12  Ounces  —  96  Drams  =  5,760  Grains 
Troy  =  1  Pound  =  12  Ounces  —  240  Pwts.  =  5,760  Grains 

The  GRAIN  is  of  the  same  weight  in  all  three  tables.     Photographic 
formulae  are  compounded  by  AVOIRDUPOIS. 

WEIGHTS — ENGLISH  —  METRIC 

27.34  Grains  =  1  Dram  =  1.77  Grams 

16  Drams  =  1  Ounce  =  28.3  Grams 

16  Ounces  =  1  Pound  =  453  Grams 

WEIGHTS — METRIC  —  ENGLISH 

1  Gram  =  .035  Ounces  =  15.4  Grains 

1  Kilogram  =  2.2046  Pounds  =  15,432  Grains 

LIQUID  MEASURE — ENGLISH  =  METRIC 

60  Minims  =  1  Dram  =  3.7  Cubic  cm. 

8  Drams  =  1  Ounce  =  29.6  Cu.  cm. 

16  Ounces  =  1  Pint  =  .473  Liters 

LIQUID  MEASURE — METRIC  —  ENGLISH 

1  Cu.  cm.  • —  .27  Drams 

1,000  Cu.  cm.  =1  Liter  =  33.8  Ounces 

1  Liter,  also  =  =  1.056  Quarts 

LAND  MEASURE  NAUTICAL  MEASURE 

7.92  Inches  =  1  Link  6  Feet  =  1  Fathom 

25  Links  =  1  Pole  120  Fathoms  =  1  Cable   length 

4  Poles  =  1  Chain  7£  Cable  Lengths  =    1  Mile 

10  Chains  =  1  Furlong  6088  Feet  =  1  Knot 

8  Furlongs  =  1  Mile  5280  Feet  =  1  Statute  Mile 

MISCELLANEOUS 

«  =  3.1416  =  Log  .499715  4jtr2  =  The  surface  of  a  sphere 

jtr2  =     The  area  of  a  circle  %^  =  The  volume  of  a  sphere 


Military  Topography  and  Photography  297 


TABLE  II 

TRIGONOMETRIC  FORMULAE 
Solution  of  Right  Triangles— 

a  1  b 

Sin  A  =      -       = — — r-  Sin  B  = 


c        —  esc  A.  c       ~  esc  B. 

b  1  a  1 


c        =  S-A.  c       -secB. 

TanA  =      S        =  c^TA7  TanB=     ;       =^±5 

K   (area)  =  ~    ab. 


c2  sin  2A 

c2  sin  2B 

4 
a2  cot  A 

4 

b2  cot  B 

2 
a2  tan  B 

2 

b2  tan  A 

2 

2 

(c  -f-  a)   (c  —  a) 

2 

1 

(c  +  b)    (c  — b) 


:  a  o •  *       • —  */     -%/  o 


Solution  of  Oblique  Triangles  — 

(1)     Given  a  side  and  any  two  angles: 

b  sin  A  c  sin  A 


sin  A 


(2)  Given  two  sides  and  their  included  angle: 

Tan  1/3  (A  -\-  B) 

Tan  1/3  (A  —  B) 

Tan  1/3  (A  -f  C) 

Tan  1/2  (A  —  C) 

Tan  1/2  (B  -|-  C) 

Tan  1/2  (B  —  C) 

(3)  Given  the  three  sides: 


1  /    (s-a)    (s  — b)    (s  -  c) 

Tan   y,  A   =— • 


,   (s  -  a)    (s  -  b)    (s  -  c) 
Tan    %B  — ir~"  •%/—          — ^ — 


(s  —  a)    (s  —  b)    (s  —  c) 


298 


Military  Topography  and  Photography 


Given  two  sides  and  the  angle  opposite  one  of  them: 
a  sin  B          a  sin  C 


Sin  A  = 


Sin  B  = 


Sin  C  = 


b  sin  A 


c  sin  A 


b 


(5)     Area: 

K  =  -g-( Where  b  =  base  and  h  —  altitude), 
be  sin  A         ca   sin   B         ab   sin    C 


2 


a2  sin  B  sin  C 

2  sin  A          = 


b2  sin  C  sin  A 
2  sin  B          = 


c2  sin  A  sin  B 
2  sin  C 


=   Vs  (s  —  a)   (s  —  b)    (s  —  c). 

The  following  Trigonometric  Formulae  will  be  found  con- 
venient in  the  solution  of  Oblique  Triangles  where  one  of  the 
angles  is  greater  than  90°  : 


Sin  (90°  +  A)  =  Cos  A 
Tan  (90°  -f-  A)  —  —  Cot  A 
Sec  (90°  +  A)  =  —  Csc  A 


Cos  (90°  -f  A)  =  —  Sin  A 
Cot  (90°  -f-  Aj i  =  —  Tan  A 
Csc  (90°  -f-  A)  —  Sec  A 


The  following  natural  trigonometric  functions  of  common 
angles  are  very  useful  in  chaining  where  it  is  desired  to  lay  off 
a  simple  angle  for  an  offset  to  pass  an  obstructing  point  or 
area.  In  expressing  the  trigorfometric  function  of  any  angle 
fractionally,  the  numerator  is  always  taken  as  unity  while  tho 
proper  value  as  given  below  is  taken  as  the  denominator. 


Sin 

Cos 

Tan 

Cot 

Sec 

Csc 

30° 

.500 

.866 

.577 

1.732 

2.000 

1.155 

45° 

.707 

1.000 

1.000 

1.000 

1.414 

1.414 

60° 

.866 

.500 

1.732 

.577 

1.155 

2.000 

90° 

1.000 

000 

00 

000 

00 

1.000 

120° 

.866 

—  .500 

—  1J32 

—  .577 

—2.000 

—1.155 

135° 

.707 

-  .707 

—1.000 

—1.000 

1.414 

1.414 

150° 

.500 

—  .866 

—  .577 

—1.732 

1.155 

2.000 

180° 

000 

—1.000 

000 

OQ 

—1.000 

00 

Military  Topography  and  Photography  299 


TABLE  III 

LOGARITHMS  or  NUMBERS 
N.         0  1  2  3  4  5 


10 

0000 

0043 

0086 

0128 

0170 

0211 

0253 

0293 

0334 

0374 

11 

0414 

0453 

0492 

0531 

0569 

0607 

0645 

0682 

0719 

0755 

12 

0792 

0828 

0864 

0899 

0934 

0969 

1004 

1038 

1072 

1106 

13 

1139 

1173 

1206 

1239 

1271 

1303 

1335 

1367 

1399 

1430 

14 

1461 

1492 

1523 

1553 

1584 

1614 

1644 

1673 

1703 

1732 

15 

1761 

1790 

1818 

1847 

1875 

1903 

1931 

1959 

1987 

2014 

16 

2041 

2068 

2095 

2122 

2148 

2175 

2201 

2227 

2253 

2279 

17 

2304 

2330 

2355 

2380 

2405 

2430 

2455 

2480 

2504 

2529 

18 

2553 

2577 

2601 

2625 

2648 

2672 

2695 

2718 

2742 

2765 

19 

2786 

2810 

2833 

2856 

2878 

2900 

2923 

2945 

2967 

2989 

20 

3010 

3032 

3053 

3075 

3096 

3118 

3139 

3160 

3181 

3201 

21 

3222 

3243 

3263 

3284 

3304 

3324 

3345 

3365 

3385 

3404 

22 

3424 

3444 

3464 

3483 

3502 

3522 

3541 

3560 

3579 

3598 

23 

3617 

3636 

3655 

3674 

3692 

3711 

3729 

3747 

3766 

3784 

24 

3802 

3820 

3838 

3856 

3874 

3892 

3909 

3927 

3945 

3962 

25 

3979 

3997 

4014 

4031 

4048 

4065 

4082 

4099 

4116 

4133 

26 

4150 

4166 

4183 

4200 

4216 

4232 

4249 

4265 

4281 

4298 

27 

4314 

4330 

4346 

4362 

4378 

4393 

4409 

4425 

4440 

4456 

28 

4472 

4487 

4502 

4518 

4533 

4548 

4564 

4579 

4594 

4609 

29 

4624 

4639 

4654 

4669 

4683 

4698 

4713 

4728 

4742 

4757 

30 

4771 

4786 

4800 

4814 

4829 

4843 

4857 

4871 

4886 

4900 

31 

4914 

4928 

4942 

4956 

4969 

4983 

4997 

5011 

5024 

5038 

32 

5052 

5065 

5079 

5092 

5105 

5119 

5132 

5145 

5159 

5172 

33 

5185 

5198 

5211 

5224 

5237 

5250 

5263 

5276 

5289 

5302 

34 

5315 

5328 

5349 

5353 

5366 

5378 

5391 

5403 

5416 

5428 

35 

5441 

5453 

5465 

5478 

5490 

5502 

5515 

5527 

5539 

5551 

36 

5563 

6575 

5587 

5599 

5611 

5623 

5635 

5647 

5658 

5670 

37 

5682 

5694 

5705 

5717 

5729 

5740 

5752 

5763 

5775 

5786 

38 

5798 

5809 

5821 

5832 

5843 

5855 

5866 

5877 

5888 

5900 

39 

5911 

5922 

5933 

5944 

5955 

5966 

5977 

5988 

5999 

6010 

40 

6021 

6031 

6042 

6053 

6064 

6075 

6085 

6096 

6107 

6117 

41 

6128 

6138 

6149 

6160 

6170 

6180 

6191 

6201 

6212 

6222 

42 

6232 

6243 

6353 

6263 

6274 

6284 

6294 

6304 

6314 

6325 

43 

6335 

6345 

6355 

6365 

6375 

6385 

6395 

6405 

6415 

6425 

44 

6435 

6444 

6454 

6464 

6474 

6484 

6493 

6503 

6513 

6522 

45 

6532 

6542 

6551 

6561 

6571 

6580 

6590 

6599 

6609 

6618 

46 

6628 

6637 

6646 

6656 

6665 

6675 

6684 

6693 

6702 

6712 

47 

6721 

6730 

6739 

6749 

6758 

6767 

6776 

6785 

6794 

6803 

48 

6812 

6821 

6830 

6839 

6848 

6857 

6866 

6875 

6884 

6893 

49 

6902 

6911 

6920 

6928 

6937 

6946 

6955 

6964 

6972 

6981 

50 

6990 

6998 

7007 

7016 

7024 

7033 

7042 

7050 

7059 

7067 

51 

7076 

7084 

7093 

7101 

7110 

7118 

7127 

7135 

7143 

7152 

52 

7160 

7168 

7177 

7185 

7193 

7202 

7210 

7218 

7226 

7235 

53 

7243 

7251 

7259 

7267 

7275 

7284 

7292 

7300 

7308 

7316 

54 

7324 

7332 

7340 

7348 

7360 

7364 

7372 

7380 

7388 

7396 

300  Military  Topography  and  Photography 

LOGARITHMS  OF  NUMBEBS — Continued 


N. 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

55 

7404 

7412 

7419 

7427 

7435 

7443 

7451 

7459 

7466 

7474 

56 

7482 

7490 

7497 

7505 

7513 

7520 

7528 

7536 

7543 

7551 

57 

7559 

7566 

7574 

7582 

7589 

7597 

7604 

7612 

7619 

7627 

58 

7634 

7642 

7649 

7657 

7664 

7672 

7679 

7686 

7694 

7701 

59 

7708 

7716 

7723 

7731 

7738 

7745 

7752 

7760 

7767 

7774 

60 

7782 

7789 

7796 

7803 

7810 

7818 

7825 

7832 

7839 

7846 

61 

7853 

7860 

7868 

7875 

7882 

7889 

7896 

7903 

7910 

7917 

62 

7924 

7931 

7938 

7945 

7952 

7959 

7966 

7973 

7980 

7987 

63 

7993 

8000 

8007 

8014 

8021 

8028 

8035 

8041 

8048 

8055 

64* 

8062 

8069 

8075 

8082 

8089 

8096 

8102 

8109 

8116 

8122 

65 

8129 

8136 

8142 

8149 

8156 

8162 

8169 

8176 

8182 

8189 

66 

8195 

8202 

8209 

8215 

8222 

8228 

8235 

8241 

8248 

8254 

67 

8261 

8267 

8274 

8280 

8287 

8293 

8299 

8306 

8312 

8319 

68 

8325 

8331 

8338 

8344 

8351 

8357 

8363 

8370 

8376 

8382 

69 

8388 

8395 

8401 

8407 

8414 

8420 

8426 

8432 

8439 

8445 

70 

8451 

8457 

8463 

8470 

8476 

8482 

8488 

8494 

8500 

8506 

71 

8513 

8519 

8525 

8531 

8537 

8543 

8549 

8555 

8561 

8567 

72 

8573 

8579 

8585 

8591 

8597 

8603 

8609 

8615 

8621 

8627 

73 

8633 

8639 

8645 

8651 

8657 

8663 

8669 

8675 

8681 

8686 

74 

8692 

8698 

8704 

8710 

8716 

8722 

8727 

8733 

8739  * 

8745 

75 

8751 

8756 

8762 

8768 

8774 

8779 

8785 

8791 

8797 

8802 

76 

8808 

8814 

8820 

8825 

8831 

8837 

8842 

8848 

8854 

8859 

77 

8865 

8871 

8876 

8882 

8887 

8893 

8899 

8904 

8910 

8915 

78 

8921 

8927 

8932 

8938 

8943 

8949 

8954 

8960 

8965 

8971 

79 

8976 

8982 

8987 

8993 

8998 

9004 

9009 

9015 

9020 

9025 

80 

9031 

9036 

9042 

9047 

9053 

9058 

9063 

9069 

9074 

9079 

81 

9085 

9090 

9096 

9101 

9106 

9112 

9117 

9122 

9128 

9133 

82 

9138 

9143 

9149 

9154 

9159 

9165 

9170 

9175 

9180 

9186 

83 

9191 

9196 

9201 

9206 

9212 

9217 

9222 

9227 

9232 

9238 

84 

9243 

9248 

9253 

9258 

9263 

9269 

9274 

9279 

9284 

9289 

85 

9294 

9299 

9304 

9309 

9315 

9320 

9325 

9330 

9335 

9340 

86 

9345 

9350 

9355 

9360 

9365 

9370 

9375 

9380 

9385 

9390 

87 

9395 

9400 

9405 

9410 

9415 

9420 

9425 

9430 

9435 

9440 

88 

9445 

9450 

9455  . 

9460 

9465 

9469 

9474 

9479 

9484 

9489 

89 

9494 

9499 

9504 

9509 

9513 

9518 

9523 

9528 

9533 

9538 

90 

9542 

9547 

9552 

9557 

9562 

9566 

9571 

9576 

9581 

9586 

91 

9590 

9595 

9600 

9605 

9609 

9614 

9619 

9624 

9628 

9633 

92 

9638 

9643 

9647 

9652 

9657 

9661 

9666 

9671 

9675 

9680 

93 

9685 

9690 

9694 

9699 

9703 

9708 

9713 

9717 

9722 

9727 

94 

9731 

9736 

9741 

9745 

9750 

9754 

9759 

9764 

9768 

9773 

95 

9777 

9782 

9786 

9791 

9795 

9800 

9805 

9809 

9814 

9818 

96 

9823 

9827 

9832 

9836 

9841 

9845 

9850 

9854 

9859 

9863 

97 

9868 

9872 

9877 

9881 

9886 

9890 

9895 

9899 

9903 

9908 

98 

9912 

9917 

9921 

9926 

9930 

9934 

9939 

9943 

9948 

9952 

99 

9956 

9961 

9965 

9969 

9974 

9978 

9983 

9987 

9991 

9995 

Military  Topography  and  Photography 


301 


TABLE  IV 

STADIA  REDUCTIONS  FOR  100 


10' 


20' 


30' 


40' 


50' 


0° 

H 

100.00 

100.00 

100.00 

99.99 

99.99 

99.98 

V 

.00 

.29 

.58 

.87 

1.16 

1.45 

1° 

H 

99.97 

99.95 

99.95 

99.93 

99.92 

99.90 

V 

1.74 

2.04 

2.33 

-     2.62 

2.91 

3.20 

2° 

H 

99.88 

99.86 

99.83 

99.81 

99.78 

99.76 

V 

3.49 

3.78 

4.07 

4.36 

4.65 

4.94 

3° 

H 

99.73 

99.69 

99.66 

99.63 

99.59 

99.56 

V 

5.23 

5.52 

5.80 

6.09 

6.38 

6.67 

4° 

H 

99.51 

99.47 

99.43 

99.38 

99.34 

99.29 

V 

6.96 

7.25 

7.53 

7.82 

8.11 

8.40 

5° 

H 

99.24 

99.19 

99.14 

99.08 

99.03 

98.97 

V 

8.68 

8.97 

S.25 

9.54 

9.83 

10.11 

6° 

H 

90.91 

98.85 

98.78 

98.72 

98.65 

98.58 

V 

10.40 

10.68 

10.96 

11.25 

11.53 

11.81 

7° 

H 

98.51 

98.44 

98.37 

98.29 

98.22 

98.14 

V 

12.10 

12.38 

12.66 

12.94 

13.22 

13.50 

8° 

H 

98.05 

97.98 

97.90 

97.82 

97.73 

97.64 

V 

13.78 

14.06 

14.34 

14.62 

14.90 

15.17 

9° 

H 

97.55 

97.46 

97.37 

97.28 

97.18 

- 

V 

15.45 

15.73 

16.00 

16.28 

16.55 

16.83 

10° 

H 

96.98 

96.88 

96.78 

96.68 

96.57 

96.47 

V 

17.10 

17.37 

17.65 

17.92 

18.19 

18.46 

11° 

H 

96.36 

96.25 

96.14 

96.03 

95.91 

95.79 

V 

18.73 

19.00 

19.27 

19.54 

19.80 

20.07 

12° 

H 

95.68 

95.56 

95.44 

95.32 

95.19 

95.07 

V 

20.34 

20.60 

20.87 

21.13 

21.39 

21.66 

13° 

H 

94.9  i 

94.81 

94.68 

94.55 

94.42 

94.28 

V 

21.92 

22.18 

22.44 

22.70 

22.96 

23.22 

14° 

H 

94.15 

94.01 

93.87 

93.73 

93.59    - 

93.45 

V 

23.47 

23.73 

23.99 

24.24 

24.49 

24.75 

3° 

6° 

9° 

12° 

15° 

c  +  f 

=    .75 

H 

.75 

.75 

.74 

.73 

.72 

V 

.05 

.08 

.12 

.16 

.20 

c  +  f 

=  1.00 

H 

1.00 

.99 

.99 

.98 

.96 

V 

.06 

.11 

.16 

.22 

.27 

c  +  f 

=rl.25 

H 

1.25 

1.24 

1.23 

1.22 

1.20 

V 

.08 

.14 

.21 

.27 

.34 

302 


Military  Topography  and  Photography 


STADIA  REDUCTIONS  FOR  100 — Continued 


0' 

15° 

H 

93.30 

V 

25.00 

16° 

H 

92.40 

V 

26.50 

17° 

H 

91.45 

V 

27.96 

18° 

H 

90.45 

V 

29.39 

19° 

H 

89.40 

V 

30.78 

20° 

H 

88.30 

V 

32.14 

21° 

H 

87.16 

V 

33.46 

22° 

H 

85.97 

V 

34.73 

23° 

H 

84.73 

V 

35.97 

24° 

H 

83.46 

V 

37.16 

25° 

H 

82.14 

V 

38.30 

26° 

H 

80.78 

V 

39.40 

27°  . 

H 

79.39 

V 

40.45 

28° 

H 

77.96 

V 

41.45 

29° 

H 

76.49 

V 

42.38 

c-f-f 

=  .75 

H 

V 

c-ff 

=  1.00 

H 

V 

c-ff 

=  1.25 

H 

V 

10' 
93.16 
25.25 
92.25 
26.74 
91.29 
28.20 
90.28 
29.62 
89.22 
31.01 
88.11 
32.36 
86.96 
33.67 
85.76 
34.94 
84.52 
36.17 
83.24 
37.35 
81.92 
38.49 
80.55 
39.58 
79.15 
40.62 
77.72 
41.60 
76.25 
42.54 

18° 
.71 
.24 
.95 
.32 
1.19 
.38 


20' 
93.01 
25.50 
92.09 
26.99 
91.12 
28.44 
90.11 
29.86 
89.04 
31.24 
87.93 
32.58 
86.77 
33.89 
85.56 
35.15 
84.31 
36.37 
83.02 
37.54 
81.69 
38.67 
80.32 
39.76 
78.92 
40.79 
77.47 
41.76 
76.00 
42.69 

21° 
.70 
.27 
.93 
.37 

1.16 
.46 


30' 
92.86 
25.75 
91.93 
27.23 
90.96 
28.68 
89.93 
30.09 
88.86 
31.47 
87.74 
32.80 
86.57 
34.10 
85.36 
35.36 
84.10 
36.57 
82.80 
37.74 
81.47 
38.86 
80.09 
39.93 
78.68 
40.96 
77.23 
41.91 
75.76 
42.85 

24° 

.68 
.31 
.91 
.41 
1.14 
.52 


40' 
92.71 
26.00 
91.77 
27.48 
90.79 
28.92 
89.76 
30.32 
88.67 
31.69 
87.54 
33.02 
86.37 
34.31 
85.15 
35.56 
83.84 
36.77 
82.58 
37.93 
81.24 
39.04 
79.86 
40.11 
78.44 
41.12 
76.98 
42.07 
75.51 
43.00 

27° 
.66 
.35 
.89 
.45 

1.11 
.58 


50' 
92.56 
26.25 
91.61 
27.72 
90.62 
29.15 
89.58 
30.55 
88.49 
31.92 
87.35 
33.24 
86.17 
34.52 
84.94 
35.76 
83.63 
36.96 
82.36 
38.11 
81.01 
39.22 
79.62 
40.28 
78.20 
41.29 
76.74 
42.23 
75.25 
43.16 

30° 
.64 
.39 
.87 
.49 

1.08 
.64 


Horizontal  Distance  =  (c  -f-  f )  cos  a  +  S  cos2  a- 
Difference  in  Elevation  =  (c-f-f)  sin  a  -}-  %  S  sin  2a. 


Military  Topography  and  Photography  303 

TABLE  V 

POLTCOXTC  PmoJtcnmn 

1"  of  1"  of  Coordinates  of  Cnrratare 

LaL  Long,  in  Lat.  in  For  1°  of  Long,  in  Meters 

Meters 


1                         3    C-2  30.713  111,303.7 

3.  /•-  30.714  111^5*4 

3    -  30.714  1114694 

4°                        30  --:  30.715  111,051.4                        67.6 

3    -;  30.715  1104994                       84.4 

30.7.5  30.716  110,7143                      101J) 

HUH  30.718  110.4^,4 

30.62  30.719 

9s                       30.54  30.721 

10°                       34  30.7B 

11                         3f.&.  30.724 

12 :                        302-5  30.727 

13  3    14  30.729  108^4854                     213 

14  31  30.731  UtyHU 

15  2<v---  30.734  1 07,5-5  l.-i- 

1                           2v73  30.737  107^035.4                     2-57,5 

17:                      2'^  30.740  MMKJ                    ^1" 

l-:                        2^42  30.743  IH^Hlfl                      2%5 -: 

19°                       2v  2'  30.746  I' 5.2'-3  ;•                     i*'"-:.' 

2v:                         2V  :-7  30.750  1  :-4.-4.- 0                      312^: 

21°                       Z84H  30.753  103^7L3                     32:2 

30.757  :.-3j.'--31  337 -:• 

30.761  102,523  4  3;> 

2-2  30.765  101,752.7                     3615 

-'•"-                         2-4  30.769  100g95Oj9                      372-3 

27-1  30.773  100,118.5 

WJB  30.777                        IMHL1 

2-                         27^2  30.782                         >-.3  2                        4O3JO 

2-                         27 .07  30.781  97,439^> 

30°                       2 , -:  30.791  96,487-0 

31 :                       2'.,>i  30.796  Ifi^Mul 

32:                        22-  30401                           -4.43-                      437jO 

33:                       HJI  30406  s3.4->3  -                     4442 

34°                        2-  30411  92^-^4                      4o-l  r 

30416  91^884 


373  2473  3-2-                          H^Bfl 

3-  244-:  30432                          *7. 

WT  24  y.  30437 

403  23  72  30448 

23^7  3   <4- 

42  23::  30453                           -2.--V  2 

43=  BLH  3    BH                         8U541.3 


45=  21  H  3-:  v-: 


304 


Military  Topography  and  Photography 


POLYCONIC  PROJECTIONS — Contiiwed 


Lat. 

46° 
47° 
48° 
49° 
50° 
51° 
52° 
53° 
54° 
55° 
56° 
57° 
58° 
59° 
60° 
61° 
62° 
63° 
64° 
65° 
66° 
67° 
68° 
69° 
70° 
71° 
72° 
73° 
74° 
75° 
76° 
77° 
78° 
79° 
80° 
81° 
82° 
83° 
84° 
85° 
86° 
87° 
88° 
89° 
90° 


1"  of 

Long,  in 

Meters 

21.52 

21.13 

20.73 

20.33 

19.92 

19.50 

19.08 

18.65 

18.22 

17.78 

17.33 

16.88 

16.43 

15.97 

15.50 

15.03 

14.56 

14.08 

13.59 

13.10 

12.61 

12.12 

11.62 

11.11 

10.61 

10.10 

9.58 

9.07 

8.55 

8.03 

7.50 

6.98 

6.45 

5.92 

5.39 

4.85 

4.32 

3.78 

3.24 

2.70 

2.16 

1.62 

1.08 

.54 

.00 


1"  of 
Lat.  in 
Meters 
30.875 
30.881 
30.886 
30.892 
30.897 
30.902 
30.908 
30.913 
30.918 
30.924 
30.929 
30.934 
30.939 
30.944 
30.948 
30.953 
30.958 
30.962 
30.967 
30.971 
30.975 
30.979 
30.983 
30.987 
30.991 
30.994 
30.997 
31.001 
31.004 
31.006 
31.009 
31.012 
31.014 
31.016 
31.018 
31.020 
31.021 
31.023 
31.024 
31.025 
31.026 
31.027 
31.027 
31.027 
31.028 


Coordinates  of 

Curvature 

For  1°  of  Long 

.  in  Meters 

"X" 

"Y" 

77,463.6 

486.3 

76,056.3 

485.4 

74,625.6 

484.0 

73,172.0 

481.9 

71,696.0 

479.3 

70,197.9 

476.1 

68,678.2 

472.3 

67,137.4 

467.9 

65,575.9 

463.0 

63,994.2 

457.5 

62,392.9 

451.4 

60,772.3 

444.8 

59,132.9 

437.6 

57,475.4 

429.9 

55,800.0 

421.7 

54,107.5 

413.0 

52,398.3 

403.8 

50,672.8 

394.0 

48,931.7 

383.8 

47,175.5 

373.1 

45,404.8 

362.0 

43,619.9 

350.4 

41,821.5 

338.4 

40,010.2 

325.9 

38,186.5 

313.1 

36,351.0 

299.9 

34,504.2 

286.4 

32,646.7 

272.4 

30,779.1 

258.2 

28,902.0 

243.6 

27,015.8 

228.8 

25,121.4 

213.6 

23,219.1 

198.2 

21,309.6 

182.5 

19,393.4 

166.7 

17,471.3 

150.6 

15,543.7 

134.3 

13,611.4 

117,9 

11,674.7 

101.3 

9,734.5 

84.6 

7,791.2 

67.8 

5,845.5 

50.9 

3,898.1 

34.0 

1,949.3 

17.0 

.0 

.0 

To  reduce  meters  to  feet:   multiply  by  3.2809- (Log.  .51600). 
From  Clark's  Ellipsoid  of  1866. 


INDEX 


Abbreviations,   Authorized    293 

Achromatic  Single  Lenses 162 

Adjustments: 

Of    level    216 

Of  Plane  Table    217 

Of    Transit    217 

Aero-Photography : 

General    Principles     196 

Rectification   of   Distortion    197 

The    Scheimpflug   Camera    198 

Scheimpflug-Kammerer        Perspecto- 

graph     199 

Aids      to      Navigation,       Conventional 

Signs  of 287 

Alidade    Ruler    84 

Anas'tigmat   Lenses     164^ 

Aneroid  Barometer,  Elevations  by   ...  9 

Astigmatism   of   Lenses    162 

Astronomical    Terms    248 

Automatic   Shutters    166 

Azimuth,    Defined     5 

Azimuth,    Back,    Defined    5 

Azimuth  by   Polaris   Observation    ....  246 

Azimuth  by   Solar  Attachment    261 

Azimuth  by  Sun   Observation    .....     .  248 

Astronomical    Terms     248 

Time    249 

General    Principles .  250 

Parallax  and  Refraction    251 

Declination,  Astronomical    „ 251 

'  Method  of  Observation    252 

Field    Notes    254 

Computations    255 

Back-Azimuth      5 

Base  Line: 

Geodetic     44 

For   Military    Surveys 57 

For  Combined  Outpost  Sketching   .  .  150 

For  Combined  Position   Sketching    .  147 

For    Individual     Outpost     Sketching  141 

For   Individual    Position-  Sketching.  143 

Base  Line  Measurement    238 

Computation      , 239 

Record     239 

Base  of  Standardization    227 

Bearing.   Defined    5 

Black    Streaks    or    Blotches    on    Nega- 
tives       193 

Blue    Printing    266 

Boundaries.    Conventional   Signs   of    .  .  276 

Buildings,  Conventional  Signs  of   ....  272 

Buildings,    Plotting    94 

Cameras 156,  170 

Camera,    Using   the    172 

Carbon   Copying  of  Maps    265 

Chaining      206 

Changes   Between    Slopes,    Plotting  of,  91 
Character     of     the     Terrain,     Plotting 

of    91,  133 

Civil   Maps    1 

Cleanliness  in  Developing 176 

Combined   Outpost   Sketching    150 

Combined  Position   Sketching 147 

Combined   Road   Sketching    145 

Compound  Shutters 166 

Computations  for: 

Base   line    239 

Sun    Azimuth    255 

Traverse  Sheet    (Control)    236 

ConcaVe  Slopes,  Plotting  of 91 


Conformation   of    Ground    13 

By    Contours    13 

By   Hachures    13 

By   Relief    13 

Constants   of  Tape,   Determination: 

Of    Temperature     228 

Of  Tension 228 

Of    Sag    228 

Construction  of  Reading  Scale 21 

Construction  of  Slope  Scale 2? 

Construction  of  Working  Scale 128 

Contours,    Conventional    Signs    278 

Contours,  Defined 13 

Control  and  Care  of  Shutters 167 

Control  Work  of  Military  Surveys   ...  52 
(1)     With     Geodetic     Triangulation 

Stations     . 52 

Preliminary  Reconnaissance   .  .  54 

Primary   Triangulation    54 

(2)  Without  Geodetic  Triangulation 

Stations     56 

Preliminary   Triangulation    ...  57 

Primary   Triangulation    57 

Secondary    Triangulation    57 

Tertiary    Triangulation    58 

Traverses,  Control    58 

Triangulation    Leveling    58 

Conventional     Signs     272 

Convex   Slopes,    Plotting    90 

Coordinate    Lines,    Map    Reproduction 

by 268 

Coordinate  Lines,   Plotting 62 

Critical  Points    79 

Location   by   Radiation    79 

Location    by    Intersection    80 

Location  by  Resection 81 

Location  by  Meandatipn    81 

Elevation    Determination     81 

Dark    Room    174 

Dark  Room,  Lighting  of 174 

Daylight  Development    .  .  . 179 

Declination,  Astronomical    251 

Declination    Computation     262 

Declination,    Magnetic    4 

Defects  in  Negatives    192 

Defects    in    Prints    195 

Definition    of   Lenses    161 

Depth  of  Focus  of  Lenses 161 

Developer      176 

Developing: 

In  Dark  Room: 

Normal  Procedure    178 

Overexposed   Plates    179 

Underexposed  Plates    179 

Daylight,  of  Film  Tank  Developer.  .  179 

Prints    189 

Direction      

Methods  of  Expression    

Magnetic    Declination     

Bearing,  Defined 

Azimuth.   Defined    

Back-Azimuth.    Defined    

Methods   of  Measurement    

Blotting 

Distance    

Methods  of  Expression    

Methods   of   Measurement    

Ground  Distance    

Map    Distance    

Slope  Distance 


3 
3 
4 
5 
5 
5 
6 

124 
7 
7 
8 


306 


Military  Topography  and  Photography 


By    Stereo-Comparator    118 

Plotting     126 

Distortion,  Rectification  of 197 

Dividers  for  Plotting  Distance 86 

Drainage,  Convention  Signs 277 

Drying    Negatives    188 


Drying   Prints    190 

Dry  Plates 


168 


Elevations,   Difference   in    8 

In  Plane  Table  Operations 81 

In    Photo-Topographic   Operations.  .  121 

Methods  of  Expression    8 

Methods  of  Measurement    9 

Of  Critical  Points,  Determination.  .  81 
Of  Instrument  Stations,  Determina- 
tion      81 

Even  Slopes,  Plotting 89 

Field  Notes  of: 

Base  Line     239 

Control  Traverse 234 

Leveling     226 

Needle    Traverse     ". 237 

Sketching    ' 100 

Sun   Azimuth    254 

Film  Tank  Developer    179 

Fixing  Negatives    186 

Fixing    Prints     199 

Focal  Length  of  Lenses    166 

Focal  Plane  Shutters    178 

Geodesy 44 

Geodetic    Operations    44 

The  Base  Line 44 

Latitude    Determination    46 

Longitude   Determination    48 

Triangulation    48 

Geodetic  Triangulation 48 

Graphic    Representation    16 

Buildings,  Towns,  Etc 18 

Lines  of  Communications    18 

Streams,  Lakes,  Etc 18 

Vegetation     18 

Graphic    Scales    19 

Ground    Distance     11 

Hachures,    Defined    13 

High  Lights    191 

Horizontal   Distance    10 

Hydrography,    Conventional    Signs     ...283 

Inclined   Readings,    Stadia    212 

Instrument  Stations 64 

Location  by  Resection 64 

Lining  In 75 

Ranging  In 75 

The  One  Point  Problem    76 

The  Three  Point  Problem 67 

The  Two  Point  Problem 64 

Location    by   Meandation    78 

Elevation,  Determination 81 

Intensification  of  Negatives    191 

Interpretation  of  Maps    38 

Intersection,   Plane  Table    80,  127 

Intersection,    Photo-Topographic    ....  109 

Lack  of  Sharpness  in  Negatives 192 

Land        Classification,        Conventional 

Signs  of    281 

Latitude  Determination    46,  258 

Lenses : 

Achromatic  Single  Lenses 162 

Anastigmat   Lenses    164 

Astigmatism    of    162 

Definition    of    161 

Depth  of  Focus  of   . 161 

Focal  Length  of 159 

Meniscus   Form    162 

Optics    of    159 

Piano-Convex    Form     162 

Rapid    Rectilinear    163 


Single  Lenses 162 

Speed  of 160 

Level  Adjustments    216 

Leveling,  Precise  Spirit 221 

General   Discussion    223 

Leveling  Terms 223 

Procedure     225 

Field  Notes 226 

Limit   of    Error    226 

Leveling,    Triangulation    58 

Lettering     293 

Lines     of     Communications,     Conven- 
tional Signs  of 274 

Lines  of  Communications,   Plotting   ;  .  93 

Lining    In     75 

Little  Contrast  in  Negatives 192 

Longitude  Determination   48,  260 

Maps: 

Civil     1 

Classes  of 1 

Definition    of    1 

Interpretation    of     38 

Military     1 

Plane 1 

Systematic  Reading  of 41 . 

War  Game  and  Fortress 1 

Map    Distance    11 

Map    Enlargement    267 

Map  Position,  Location  of 28 

By  Two  Plotted  Points 28 

By  Three  Plotted  Points 28 

By  Ranging  In    30 

By   Lining   In    30 

Map  Reproduction    265 

Blue  Printing 266 

Carbon  Copying 265 

Lithographic    267 

Photographic     267 

Tracing    265 

White    Printing    266 

Meandation   (Traversing)    135,  230 

Measurement   of    Angle    by    Repetition  241 

Measurement  of  Angles  in  Series  ....  241 

Measuring  and  Plotting  Direction.  .84,  124 

Measuring  and  Plotting  Distance.  .86,  126 

Measuring  and  Plotting  Slopes  ...  88,  131 

Memory    Sketching     144 

Meniscus    Lenses    162 

Military,  Conventional  Signs 289 

Military    Maps    1 

Military  Photography 196 

Monuments,       Survey,       Conventional 

Signs    of 276 

Mottled  Appearance  in  Negatives  ....  195 

Mounting   Prints    190 

Much  Contrast  in  Negatives 192 

Natural  Character  of  Terrain    91 

One  Point  Problem    76 

Opaque  Lines  in  Negatives    194 

Opaque   Spots  in  Negatives    194 

Optics  of  Lenses    159 

Orientation  of  Maps    

By    Comparison    27 

By    Compass     24 

By    North    Star    25 

By    Watch    27 

By  the   Sun    24 

Orientation  of  Picture  Trace    117 

Outpost   Sketches    2 

Outpost  Sketching,  Combined 150 

Outpost   Sketching,  Individual    143 

Outpost  Views    196 

Overexposed  Plates    179 

Pantagraph 267 

Parallax,   Corrected    220 

Parallax.    TVoflnfid     .  251 


Military  Topography  and  Photography 


307 


Photographic    Intersection     109 

Photographic    Map    Enlargement    ....  267 

Photographic  Resection    113 

Photographic    Terms    191 

Photography      156 

Photography,    Military     196 

Photo-Theodolite     102 

Photo-Topographic    Control : 

Orientation  of  Picture  Trace 117 

Photo    Intersection     109 

Photo    Resection     113 

Stereo-Comparator,  Distance  by   ...  118 

Elevation    Determination     121 

Photo-Topographic  Instruments: 

Camera  and  Accessories    102 

Photo-Theodolite    102 

Stereo-Autograph    104 

Stereo-Comparator .  104 

Pin  Holes  i»  Negatives 194 

Place    Sketches     2 

Place,   or   "Eye,"    Sketching    143 

Place  and  Reconnaissance  Views    .  .  .  19£ 

Plane  Table  Adjustments    217 

Plane  Table  Operations 58 

Plane   Maps    1 

Piano-Convex    Lenses    162 

Plate    Holders    169 

Plates,    Dry    168 

Plotting: 

Coordinate    Lines     62 

Character  of  Terrain    91,  133 

Direction 84,  124 

Distance 86,  126 

Slopes    88,  131 

Triangulation    Stations    62 

Polvconic    Projections    268 

Position  and  Outpost  Views 196 

Position    Sketching,    Combined    147 

Position    Sketching,    Individual    141' 

Preparation  of  Field  Sheets 60 

Printing,   Photographic    187 

Printing    Papers    187 

Printing    Light     188 

Developing  and  Fixing    189 

Washing   and   Drying    190 

Mounting     190 

Protractor  for  Plotting 84 

Radiation      79 

Ranging    In     75 

Rapid  Rectilinear  Lenses    163 

Rapid    Sketching    152 

Reading  Scales    19,    86 

Reading   Scales,    Construction   of    ....  24 

Reconnaissance    Views    196 

Reduction    of   Negative    191 

Refraction,    Denned     251 

Relief   Maps    13 

Representative    Fraction     18 

Resection    64,  82,  127 

Resection,    Photographic    113 

Road    Sketching,    Combined    145 

Road   Sketching,   Individual    135 

Roll    Films    170 

Sag  of  Steel  Tape .  228 

Scale   of  Maps    18 

Rv  Representative  Fraction   .  .  .  .  .  18 

By  Words  and  Figures    19 

By   Graphic   Representation    ....  19 

Reading    Scales    19 

Slope  Scales    19 

Working   Scales 19 

Construction  of  Reading  Scale   .  ,  21 

Construction  of  Slope  Scale    22 

Construction    of    Working    Scale    .  .  128 


Normal  System,  U.  S.  Army 21 

Scheimpflug   Camera    198 

Scheimpflug-Kammerer   Perspecto- 

graph 199 

Secondary    Triangulation     57 

Setting  Up  Plane  Table   63 

Shadows  in  Negatives 191 

Shutters: 

Simple    Shutters    166 

Automatic     Shutters     166 

Focal   Plane   Shutters    167 

Control  and  Care  of 167 

Single    Lenses    162 

Sketches: 

Area    2 

Outpost     2 

Place     2 

Position     2 

Road     1 

Sketching: 

Combined    Outpost    Sketching    ....  150 

Combined    Position    Sketching    ....  147 

Combined    Road    Sketching    145 

Concrete  Example  of 94 

Field    Notes    100 

Individual    Outpost    143 

Individual  Position    141 

Individual   Road    Sketching    135 

Memory    Sketching     144 

Place  or  "Eye"  Sketching 143 

Rapid     Sketching     152 

Slopes : 

Even  Slopes  Plotting 89 

Convex  Slopes  Plotting 90 

Concave    Slopes    Plotting    91 

Changes  between  Slopes    91 

Slope    Distance     8 

Slope    Scales     19 

Slope  Scales,  Construction  of 22 

Speed   of   Lenses    160 

Spreading    of    High    Lights    in    Nega- 
tives       193 

Strains  on  Negatives 193 

Stadia: 

Constant,    Determination     229 

Inclined  Readings    212 

Reductions    132 

Theory  of    210 

Standardization  of  Steel  Tape    226 

Stereo-Autograph      104 

Stereo-Comparator     104 

Streams,    Conventional    Signs    279 

Streams,    Plotting    18,  133 

Streams,    Sketching    of    97 

Structures,  Conventional  Signs 272 

Systematic  Reading  of  Maps 40 

Telemetric    Measurement    of    Distance  210 

Temperature  Constant  of  Steel  Tape.  .  228 

Tension   Constant   of   Steel  Tape    ....  228 

Tertiary    Trian dilation     58 

Theory  of  Stadia    210 

Three  Point  Problem: 

Plane   Table    Resection    67 

Photographic    Resection     113 

Timing   the    Exposure    174 

Too  Dense  a  Negative 192 

Too  Thin  a  Nesative 192 

Topographical   Maps    1,  2 

Topographic  Methods 59 

Topo-Photographv 196 

Topographical    Surveying    43 

Towns  and  Buildings: 

Conventional    Signs    of    272 

Plotting  of 94 


308  Military  Topography  and  Photography 

Tracing  Maps 265        Vegetation,   Plotting  of    93 

Transit   Adjustments    217  Verniers: 

Transparent   Lines  in   Negatives    ....  194             Direct   Verniers    213 

Transparent  Spots  in  Negatives 194             Double    Verniers    215 

Traverses:                                                                             Folded    Verniers     216 

Back-Sight   Traverse    237             Least  Count  of  Vernier    215 

Control   Traverse    230            Retrograde    Verniers    215 

Field    Notes     234        Vertical   Angle   Measurement    131 

Needle    Traverse 237        Vertical   Interval    10 

Procedure 230  Views: 

Traverse    Sheet     236             Place  and  Reconnaissance 196 

Triangles  for  Plotting 85             Position    and    Outpost     .  . 196 

Triangulation :  Visibility : 

Geodetic     48            Of   Areas    38 

Leveling 58             Of  Points    31 

Photo-Topographic     104        War  Game  and  Fortress  Maps 1 

Primary     54,  57        Washing   Plates    187 

Secondary     57        Washing  Prints    190 

Tertiary     58        White    Printing    . , 266 

Triangulation   Stations,    Plotting    62        Words  and  Figures,   Scale  by    19 

T.    Square    and    Protractor    for    Plot-                Working   Scales    19 

ting     84        Working   Scales,    Construction    128 

Underexposed    Plates     179        Zenith     248 


UNIVEESITY  OF  CALIFOENIA  LIBRARY 
BERKELEY 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
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JAN  8  1920 


OCT  26J82S 


APR  2  S 

DEAD 


50m-7,'16 


YC  5G892 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


